# Function and Expression

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## Arithmetic Operators and Functions

### Arithmetic Operators

#### Unary Arithmetic Operators

Supported operators: `+`, `-`

Supported input data types: `INT32`, `INT64` and `FLOAT`

Output data type: consistent with the input data type

#### Binary Arithmetic Operators

Supported operators: `+`, `-`, `*`, `/`, `%`

Supported input data types: `INT32`, `INT64`, `FLOAT` and `DOUBLE`

Output data type: `DOUBLE`

Note: Only when the left operand and the right operand under a certain timestamp are not  `null`, the binary arithmetic operation will have an output value.

#### Example

```sql
select s1, - s1, s2, + s2, s1 + s2, s1 - s2, s1 * s2, s1 / s2, s1 % s2 from root.sg.d1
```

Result:

```
+-----------------------------+-------------+--------------+-------------+-------------+-----------------------------+-----------------------------+-----------------------------+-----------------------------+-----------------------------+
|                         Time|root.sg.d1.s1|-root.sg.d1.s1|root.sg.d1.s2|root.sg.d1.s2|root.sg.d1.s1 + root.sg.d1.s2|root.sg.d1.s1 - root.sg.d1.s2|root.sg.d1.s1 * root.sg.d1.s2|root.sg.d1.s1 / root.sg.d1.s2|root.sg.d1.s1 % root.sg.d1.s2|
+-----------------------------+-------------+--------------+-------------+-------------+-----------------------------+-----------------------------+-----------------------------+-----------------------------+-----------------------------+
|1970-01-01T08:00:00.001+08:00|          1.0|          -1.0|          1.0|          1.0|                          2.0|                          0.0|                          1.0|                          1.0|                          0.0|
|1970-01-01T08:00:00.002+08:00|          2.0|          -2.0|          2.0|          2.0|                          4.0|                          0.0|                          4.0|                          1.0|                          0.0|
|1970-01-01T08:00:00.003+08:00|          3.0|          -3.0|          3.0|          3.0|                          6.0|                          0.0|                          9.0|                          1.0|                          0.0|
|1970-01-01T08:00:00.004+08:00|          4.0|          -4.0|          4.0|          4.0|                          8.0|                          0.0|                         16.0|                          1.0|                          0.0|
|1970-01-01T08:00:00.005+08:00|          5.0|          -5.0|          5.0|          5.0|                         10.0|                          0.0|                         25.0|                          1.0|                          0.0|
+-----------------------------+-------------+--------------+-------------+-------------+-----------------------------+-----------------------------+-----------------------------+-----------------------------+-----------------------------+
Total line number = 5
It costs 0.014s
```

### Arithmetic Functions

Currently, IoTDB supports the following mathematical functions. The behavior of these mathematical functions is consistent with the behavior of these functions in the Java Math standard library.

| Function Name | Allowed Input Series Data Types | Output Series Data Type       | Required Attributes                                          | Corresponding Implementation in the Java Standard Library    |
| ------------- | ------------------------------- | ----------------------------- | ------------------------------------------------------------ | ------------------------------------------------------------ |
| SIN           | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#sin(double)                                             |
| COS           | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#cos(double)                                             |
| TAN           | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#tan(double)                                             |
| ASIN          | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#asin(double)                                            |
| ACOS          | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#acos(double)                                            |
| ATAN          | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#atan(double)                                            |
| SINH          | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#sinh(double)                                            |
| COSH          | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#cosh(double)                                            |
| TANH          | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#tanh(double)                                            |
| DEGREES       | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#toDegrees(double)                                       |
| RADIANS       | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#toRadians(double)                                       |
| ABS           | INT32 / INT64 / FLOAT / DOUBLE  | Same type as the input series | /                                                            | Math#abs(int) / Math#abs(long) /Math#abs(float) /Math#abs(double) |
| SIGN          | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#signum(double)                                          |
| CEIL          | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#ceil(double)                                            |
| FLOOR         | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#floor(double)                                           |
| ROUND         | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | 'places' : Round the significant number, positive number is the significant number after the decimal point, negative number is the significant number of whole number | Math#rint(Math#pow(10,places))/Math#pow(10,places)           |
| EXP           | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#exp(double)                                             |
| LN            | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#log(double)                                             |
| LOG10         | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#log10(double)                                           |
| SQRT          | INT32 / INT64 / FLOAT / DOUBLE  | DOUBLE                        | /                                                            | Math#sqrt(double)                                            |

Example:

```   sql
select s1, sin(s1), cos(s1), tan(s1) from root.sg1.d1 limit 5 offset 1000;
```

Result:

```
+-----------------------------+-------------------+-------------------+--------------------+-------------------+
|                         Time|     root.sg1.d1.s1|sin(root.sg1.d1.s1)| cos(root.sg1.d1.s1)|tan(root.sg1.d1.s1)|
+-----------------------------+-------------------+-------------------+--------------------+-------------------+
|2020-12-10T17:11:49.037+08:00|7360723084922759782| 0.8133527237573284|  0.5817708713544664| 1.3980636773094157|
|2020-12-10T17:11:49.038+08:00|4377791063319964531|-0.8938962705202537|  0.4482738644511651| -1.994085181866842|
|2020-12-10T17:11:49.039+08:00|7972485567734642915| 0.9627757585308978|-0.27030138509681073|-3.5618602479083545|
|2020-12-10T17:11:49.040+08:00|2508858212791964081|-0.6073417341629443| -0.7944406950452296| 0.7644897069734913|
|2020-12-10T17:11:49.041+08:00|2817297431185141819|-0.8419358900502509| -0.5395775727782725| 1.5603611649667768|
+-----------------------------+-------------------+-------------------+--------------------+-------------------+
Total line number = 5
It costs 0.008s
```

#### ROUND
Example:
```sql
select s4,round(s4),round(s4,2),round(s4,-1) from root.sg1.d1 
```

```sql
+-----------------------------+-------------+--------------------+----------------------+-----------------------+
|                         Time|root.db.d1.s4|ROUND(root.db.d1.s4)|ROUND(root.db.d1.s4,2)|ROUND(root.db.d1.s4,-1)|
+-----------------------------+-------------+--------------------+----------------------+-----------------------+
|1970-01-01T08:00:00.001+08:00|    101.14345|               101.0|                101.14|                  100.0|
|1970-01-01T08:00:00.002+08:00|    20.144346|                20.0|                 20.14|                   20.0|
|1970-01-01T08:00:00.003+08:00|    20.614372|                21.0|                 20.61|                   20.0|
|1970-01-01T08:00:00.005+08:00|    20.814346|                21.0|                 20.81|                   20.0|
|1970-01-01T08:00:00.006+08:00|     60.71443|                61.0|                 60.71|                   60.0|
|2023-03-13T16:16:19.764+08:00|    10.143425|                10.0|                 10.14|                   10.0|
+-----------------------------+-------------+--------------------+----------------------+-----------------------+
Total line number = 6
It costs 0.059s
```

<!--

    Licensed to the Apache Software Foundation (ASF) under one
    or more contributor license agreements.  See the NOTICE file
    distributed with this work for additional information
    regarding copyright ownership.  The ASF licenses this file
    to you under the Apache License, Version 2.0 (the
    "License"); you may not use this file except in compliance
    with the License.  You may obtain a copy of the License at
    
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    specific language governing permissions and limitations
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## Comparison Operators and Functions

### Basic comparison operators

Supported operators `>`, `>=`, `<`, `<=`, `==`, `!=` (or  `<>` )

Supported input data types: `INT32`, `INT64`, `FLOAT` and `DOUBLE` 

Note: It will transform all type to `DOUBLE` then do computation. 

Output data type: `BOOLEAN`

**Example:**

```sql
select a, b, a > 10, a <= b, !(a <= b), a > 10 && a > b from root.test;
```

```
IoTDB> select a, b, a > 10, a <= b, !(a <= b), a > 10 && a > b from root.test;
+-----------------------------+-----------+-----------+----------------+--------------------------+---------------------------+------------------------------------------------+
|                         Time|root.test.a|root.test.b|root.test.a > 10|root.test.a <= root.test.b|!root.test.a <= root.test.b|(root.test.a > 10) & (root.test.a > root.test.b)|
+-----------------------------+-----------+-----------+----------------+--------------------------+---------------------------+------------------------------------------------+
|1970-01-01T08:00:00.001+08:00|         23|       10.0|            true|                     false|                       true|                                            true|
|1970-01-01T08:00:00.002+08:00|         33|       21.0|            true|                     false|                       true|                                            true|
|1970-01-01T08:00:00.004+08:00|         13|       15.0|            true|                      true|                      false|                                           false|
|1970-01-01T08:00:00.005+08:00|         26|        0.0|            true|                     false|                       true|                                            true|
|1970-01-01T08:00:00.008+08:00|          1|       22.0|           false|                      true|                      false|                                           false|
|1970-01-01T08:00:00.010+08:00|         23|       12.0|            true|                     false|                       true|                                            true|
+-----------------------------+-----------+-----------+----------------+--------------------------+---------------------------+------------------------------------------------+
```

### `BETWEEN ... AND ...` operator

|operator |meaning|
|-----------------------------|-----------|
|`BETWEEN ... AND ...` |within the specified range|
|`NOT BETWEEN ... AND ...` |Not within the specified range|

**Example:** Select data within or outside the interval [36.5,40]:

```sql
select temperature from root.sg1.d1 where temperature between 36.5 and 40;
```

```sql
select temperature from root.sg1.d1 where temperature not between 36.5 and 40;
```

### Fuzzy matching operator

For TEXT type data, support fuzzy matching of data using `Like` and `Regexp` operators.

|operator |meaning|
|-----------------------------|-----------|
|`LIKE` | matches simple patterns|
|`NOT LIKE` |cannot match simple pattern|
|`REGEXP` | Match regular expression|
|`NOT REGEXP` |Cannot match regular expression|

Input data type: `TEXT`

Return type: `BOOLEAN`

#### Use `Like` for fuzzy matching

**Matching rules:**

- `%` means any 0 or more characters.
- `_` means any single character.

**Example 1:** Query the data under `root.sg.d1` that contains `'cc'` in `value`.

```shell
IoTDB> select * from root.sg.d1 where value like '%cc%'
+--------------------------+----------------+
| Time|root.sg.d1.value|
+--------------------------+----------------+
|2017-11-01T00:00:00.000+08:00| aabbccdd|
|2017-11-01T00:00:01.000+08:00| cc|
+--------------------------+----------------+
Total line number = 2
It costs 0.002s
```

**Example 2:** Query the data under `root.sg.d1` with `'b'` in the middle of `value` and any single character before and after.

```shell
IoTDB> select * from root.sg.device where value like '_b_'
+--------------------------+----------------+
| Time|root.sg.d1.value|
+--------------------------+----------------+
|2017-11-01T00:00:02.000+08:00|abc|
+--------------------------+----------------+
Total line number = 1
It costs 0.002s
```

#### Use `Regexp` for fuzzy matching

The filter condition that needs to be passed in is **Java standard library style regular expression**.

**Common regular matching examples:**

```
All characters with a length of 3-20: ^.{3,20}$
Uppercase English characters: ^[A-Z]+$
Numbers and English characters: ^[A-Za-z0-9]+$
Starting with a: ^a.*
```

**Example 1:** Query the string of 26 English characters for value under root.sg.d1.

```shell
IoTDB> select * from root.sg.d1 where value regexp '^[A-Za-z]+$'
+--------------------------+----------------+
| Time|root.sg.d1.value|
+--------------------------+----------------+
|2017-11-01T00:00:00.000+08:00| aabbccdd|
|2017-11-01T00:00:01.000+08:00| cc|
+--------------------------+----------------+
Total line number = 2
It costs 0.002s
```

**Example 2:** Query root.sg.d1 where the value is a string consisting of 26 lowercase English characters and the time is greater than 100.

```shell
IoTDB> select * from root.sg.d1 where value regexp '^[a-z]+$' and time > 100
+--------------------------+----------------+
| Time|root.sg.d1.value|
+--------------------------+----------------+
|2017-11-01T00:00:00.000+08:00| aabbccdd|
|2017-11-01T00:00:01.000+08:00| cc|
+--------------------------+----------------+
Total line number = 2
It costs 0.002s
```

**Example 3:**

```sql
select b, b like '1%', b regexp '[0-2]' from root.test;
```

operation result
```
+-----------------------------+-----------+------- ------------------+--------------------------+
| Time|root.test.b|root.test.b LIKE '^1.*?$'|root.test.b REGEXP '[0-2]'|
+-----------------------------+-----------+------- ------------------+--------------------------+
|1970-01-01T08:00:00.001+08:00| 111test111| true| true|
|1970-01-01T08:00:00.003+08:00| 333test333| false| false|
+-----------------------------+-----------+------- ------------------+--------------------------+
```

### `IS NULL` operator

|operator |meaning|
|-----------------------------|-----------|
|`IS NULL` |is a null value|
|`IS NOT NULL` |is not a null value|

**Example 1:** Select data with empty values:

```sql
select code from root.sg1.d1 where temperature is null;
```

**Example 2:** Select data with non-null values:

```sql
select code from root.sg1.d1 where temperature is not null;
```

### `IN` operator

|operator |meaning|
|-----------------------------|-----------|
|`IN` / `CONTAINS` | are the values ​​in the specified list|
|`NOT IN` / `NOT CONTAINS` |not a value in the specified list|

Input data type: `All Types`

return type `BOOLEAN`

**Note: Please ensure that the values ​​in the collection can be converted to the type of the input data. **
> 
> For example:
>
> `s1 in (1, 2, 3, 'test')`, the data type of `s1` is `INT32`
>
> We will throw an exception because `'test'` cannot be converted to type `INT32`

**Example 1:** Select data with values ​​within a certain range:

```sql
select code from root.sg1.d1 where code in ('200', '300', '400', '500');
```

**Example 2:** Select data with values ​​outside a certain range:

```sql
select code from root.sg1.d1 where code not in ('200', '300', '400', '500');
```

**Example 3:**

```sql
select a, a in (1, 2) from root.test;
```

Output 2:
```
+-----------------------------+-----------+------- -------------+
| Time|root.test.a|root.test.a IN (1,2)|
+-----------------------------+-----------+------- -------------+
|1970-01-01T08:00:00.001+08:00| 1| true|
|1970-01-01T08:00:00.003+08:00| 3| false|
+-----------------------------+-----------+------- -------------+
```

### Condition Functions

Condition functions are used to check whether timeseries data points satisfy some specific condition. 

They return BOOLEANs.

Currently, IoTDB supports the following condition functions:

| Function Name | Allowed Input Series Data Types | Required Attributes                           | Output Series Data Type | Description                                   |
| ------------- | ------------------------------- | --------------------------------------------- | ----------------------- | --------------------------------------------- |
| ON_OFF        | INT32 / INT64 / FLOAT / DOUBLE  | `threshold`: a double type variate            | BOOLEAN                 | Return `ts_value >= threshold`.               |
| IN_RANGR      | INT32 / INT64 / FLOAT / DOUBLE  | `lower`: DOUBLE type<br/>`upper`: DOUBLE type | BOOLEAN                 | Return `ts_value >= lower && value <= upper`. |

Example Data:
```
IoTDB> select ts from root.test;
+-----------------------------+------------+
|                         Time|root.test.ts|
+-----------------------------+------------+
|1970-01-01T08:00:00.001+08:00|           1|
|1970-01-01T08:00:00.002+08:00|           2|
|1970-01-01T08:00:00.003+08:00|           3|
|1970-01-01T08:00:00.004+08:00|           4|
+-----------------------------+------------+
```

#### Test 1
SQL:
```sql
select ts, on_off(ts, 'threshold'='2') from root.test;
```

Output:
```
IoTDB> select ts, on_off(ts, 'threshold'='2') from root.test;
+-----------------------------+------------+-------------------------------------+
|                         Time|root.test.ts|on_off(root.test.ts, "threshold"="2")|
+-----------------------------+------------+-------------------------------------+
|1970-01-01T08:00:00.001+08:00|           1|                                false|
|1970-01-01T08:00:00.002+08:00|           2|                                 true|
|1970-01-01T08:00:00.003+08:00|           3|                                 true|
|1970-01-01T08:00:00.004+08:00|           4|                                 true|
+-----------------------------+------------+-------------------------------------+
```

#### Test 2
Sql:
```sql
select ts, in_range(ts, 'lower'='2', 'upper'='3.1') from root.test;
```

Output:
```
IoTDB> select ts, in_range(ts,'lower'='2', 'upper'='3.1') from root.test;
+-----------------------------+------------+--------------------------------------------------+
|                         Time|root.test.ts|in_range(root.test.ts, "lower"="2", "upper"="3.1")|
+-----------------------------+------------+--------------------------------------------------+
|1970-01-01T08:00:00.001+08:00|           1|                                             false|
|1970-01-01T08:00:00.002+08:00|           2|                                              true|
|1970-01-01T08:00:00.003+08:00|           3|                                              true|
|1970-01-01T08:00:00.004+08:00|           4|                                             false|
+-----------------------------+------------+--------------------------------------------------+
```

<!--

    Licensed to the Apache Software Foundation (ASF) under one
    or more contributor license agreements.  See the NOTICE file
    distributed with this work for additional information
    regarding copyright ownership.  The ASF licenses this file
    to you under the Apache License, Version 2.0 (the
    "License"); you may not use this file except in compliance
    with the License.  You may obtain a copy of the License at
    
        http://www.apache.org/licenses/LICENSE-2.0
    
    Unless required by applicable law or agreed to in writing,
    software distributed under the License is distributed on an
    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
    KIND, either express or implied.  See the License for the
    specific language governing permissions and limitations
    under the License.

-->

## Logical Operators

### Unary Logical Operators

Supported operator `!`

Supported input data types: `BOOLEAN`

Output data type: `BOOLEAN`

Hint: the priority of `!` is the same as `-`. Remember to use brackets to modify priority.

### Binary Logical Operators

Supported operators AND:`and`,`&`, `&&`; OR:`or`,`|`,`||`

Supported input data types: `BOOLEAN`

Output data type: `BOOLEAN`

Note: Only when the left operand and the right operand under a certain timestamp are both `BOOLEAN` type, the binary logic operation will have an output value.

**Example:**

```sql
select a, b, a > 10, a <= b, !(a <= b), a > 10 && a > b from root.test;
```

Output:
```
IoTDB> select a, b, a > 10, a <= b, !(a <= b), a > 10 && a > b from root.test;
+-----------------------------+-----------+-----------+----------------+--------------------------+---------------------------+------------------------------------------------+
|                         Time|root.test.a|root.test.b|root.test.a > 10|root.test.a <= root.test.b|!root.test.a <= root.test.b|(root.test.a > 10) & (root.test.a > root.test.b)|
+-----------------------------+-----------+-----------+----------------+--------------------------+---------------------------+------------------------------------------------+
|1970-01-01T08:00:00.001+08:00|         23|       10.0|            true|                     false|                       true|                                            true|
|1970-01-01T08:00:00.002+08:00|         33|       21.0|            true|                     false|                       true|                                            true|
|1970-01-01T08:00:00.004+08:00|         13|       15.0|            true|                      true|                      false|                                           false|
|1970-01-01T08:00:00.005+08:00|         26|        0.0|            true|                     false|                       true|                                            true|
|1970-01-01T08:00:00.008+08:00|          1|       22.0|           false|                      true|                      false|                                           false|
|1970-01-01T08:00:00.010+08:00|         23|       12.0|            true|                     false|                       true|                                            true|
+-----------------------------+-----------+-----------+----------------+--------------------------+---------------------------+------------------------------------------------+
```

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    or more contributor license agreements.  See the NOTICE file
    distributed with this work for additional information
    regarding copyright ownership.  The ASF licenses this file
    to you under the Apache License, Version 2.0 (the
    "License"); you may not use this file except in compliance
    with the License.  You may obtain a copy of the License at
    
        http://www.apache.org/licenses/LICENSE-2.0
    
    Unless required by applicable law or agreed to in writing,
    software distributed under the License is distributed on an
    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
    KIND, either express or implied.  See the License for the
    specific language governing permissions and limitations
    under the License.

-->

## Aggregate Functions

Aggregate functions are many-to-one functions. They perform aggregate calculations on a set of values, resulting in a single aggregated result.

All aggregate functions except `COUNT()`, `COUNT_IF()` ignore null values and return null when there are no input rows or all values are null. For example, `SUM()` returns null instead of zero, and `AVG()` does not include null values in the count.

The aggregate functions supported by IoTDB are as follows:

| Function Name | Description                                                  | Allowed Input Series Data Types                     | Required Attributes                                          | Output Series Data Type             |
| ------------- | ------------------------------------------------------------ |-----------------------------------------------------| ------------------------------------------------------------ | ----------------------------------- |
| SUM           | Summation.                                                   | INT32 INT64 FLOAT DOUBLE                            | /                                                            | DOUBLE                              |
| COUNT         | Counts the number of data points.                            | All data types                                      | /                                                            | INT                                 |
| AVG           | Average.                                                     | INT32 INT64 FLOAT DOUBLE                            | /                                                            | DOUBLE                              |
 STDDEV        | Alias for STDDEV_SAMP. Return the sample standard deviation. | INT32 INT64 FLOAT DOUBLE        | /                                                            | DOUBLE                              |
| STDDEV_POP    | Return the population standard deviation.                    | INT32 INT64 FLOAT DOUBLE        | /                                                            | DOUBLE                              |
| STDDEV_SAMP   | Return the sample standard deviation.                        | INT32 INT64 FLOAT DOUBLE        | /                                                            | DOUBLE                              |
| VARIANCE      | Alias for VAR_SAMP. Return the sample variance.              | INT32 INT64 FLOAT DOUBLE        | /                                                            | DOUBLE                              |
| VAR_POP       | Return the population variance.                              | INT32 INT64 FLOAT DOUBLE        | /                                                            | DOUBLE                              |
| VAR_SAMP      | Return the sample variance.                                  | INT32 INT64 FLOAT DOUBLE        | /                                                            | DOUBLE                              |
| EXTREME       | Finds the value with the largest absolute value. Returns a positive value if the maximum absolute value of positive and negative values is equal. | INT32 INT64 FLOAT DOUBLE                            | /                                                            | Consistent with the input data type |
| MAX_VALUE     | Find the maximum value.                                      | INT32 INT64 FLOAT DOUBLE                            | /                                                            | Consistent with the input data type |
| MIN_VALUE     | Find the minimum value.                                      | INT32 INT64 FLOAT DOUBLE                            | /                                                            | Consistent with the input data type |
| FIRST_VALUE   | Find the value with the smallest timestamp.                  | All data types                                      | /                                                            | Consistent with input data type     |
| LAST_VALUE    | Find the value with the largest timestamp.                   | All data types                                      | /                                                            | Consistent with input data type     |
| MAX_TIME      | Find the maximum timestamp.                                  | All data Types                                      | /                                                            | Timestamp                           |
| MIN_TIME      | Find the minimum timestamp.                                  | All data Types                                      | /                                                            | Timestamp                           |
| COUNT_IF      | Find the number of data points that continuously meet a given condition and the number of data points that meet the condition (represented by keep) meet the specified threshold. | BOOLEAN                                             | `[keep >=/>/=/!=/</<=]threshold`：The specified threshold or threshold condition, it is equivalent to `keep >= threshold` if `threshold` is used alone, type of `threshold` is `INT64` `ignoreNull`：Optional, default value is `true`；If the value is `true`, null values are ignored, it means that if there is a null value in the middle, the value is ignored without interrupting the continuity. If the value is `true`, null values are not ignored, it means that if there are null values in the middle, continuity will be broken | INT64                               |
| TIME_DURATION | Find the difference between the timestamp of the largest non-null value and the timestamp of the smallest non-null value in a column | All data Types                                      | /                                                            | INT64                               |
| MODE          | Find the mode. Note:  1.Having too many different values in the input series risks a memory exception;  2.If all the elements have the same number of occurrences, that is no Mode, return the value with earliest time;  3.If there are many Modes, return the Mode with earliest time. | All data Types                                      | /                                                            | Consistent with the input data type |
| COUNT_TIME    | The number of timestamps in the query data set. When used with `align by device`, the result is the number of timestamps in the data set per device.                                                                                                                                       | All data Types, the input parameter can only be `*` | /                                                                                                                                                                                                                                                                                                                                                                                                                           | INT64                                        |
| MAX_BY        | MAX_BY(x, y) returns the value of x corresponding to the maximum value of the input y. MAX_BY(time, x) returns the timestamp when x is at its maximum value.  <br> Note: Supported from version V1.3.2                                                                                                                           | The first input x can be of any type, while the second input y must be of type INT32, INT64, FLOAT, DOUBLE, STRING, TIMESTAMP or DATE. | /                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | Consistent with the data type of the first input x. |
| MIN_BY        | MIN_BY(x, y) returns the value of x corresponding to the minimum value of the input y. MIN_BY(time, x) returns the timestamp when x is at its minimum value.   <br> Note: Supported from version V1.3.2                                                                                                                          | The first input x can be of any type, while the second input y must be of type INT32, INT64, FLOAT, DOUBLE, STRING, TIMESTAMP or DATE. | / | Consistent with the data type of the first input x. |


### COUNT_IF

#### Grammar
```sql
count_if(predicate, [keep >=/>/=/!=/</<=]threshold[, 'ignoreNull'='true/false'])
```
predicate: legal expression with `BOOLEAN` return type

use of threshold and ignoreNull can see above table

>Note: count_if is not supported to use with SlidingWindow in group by time now

#### example

##### raw data

``` 
+-----------------------------+-------------+-------------+
|                         Time|root.db.d1.s1|root.db.d1.s2|
+-----------------------------+-------------+-------------+
|1970-01-01T08:00:00.001+08:00|            0|            0|
|1970-01-01T08:00:00.002+08:00|         null|            0|
|1970-01-01T08:00:00.003+08:00|            0|            0|
|1970-01-01T08:00:00.004+08:00|            0|            0|
|1970-01-01T08:00:00.005+08:00|            1|            0|
|1970-01-01T08:00:00.006+08:00|            1|            0|
|1970-01-01T08:00:00.007+08:00|            1|            0|
|1970-01-01T08:00:00.008+08:00|            0|            0|
|1970-01-01T08:00:00.009+08:00|            0|            0|
|1970-01-01T08:00:00.010+08:00|            0|            0|
+-----------------------------+-------------+-------------+
```

##### Not use `ignoreNull` attribute (Ignore Null)

SQL:
```sql
select count_if(s1=0 & s2=0, 3), count_if(s1=1 & s2=0, 3) from root.db.d1
```

Result:
```
+--------------------------------------------------+--------------------------------------------------+
|count_if(root.db.d1.s1 = 0 & root.db.d1.s2 = 0, 3)|count_if(root.db.d1.s1 = 1 & root.db.d1.s2 = 0, 3)|
+--------------------------------------------------+--------------------------------------------------+
|                                                 2|                                                 1|
+--------------------------------------------------+--------------------------------------------------
```

##### Use `ignoreNull` attribute

SQL:
```sql
select count_if(s1=0 & s2=0, 3, 'ignoreNull'='false'), count_if(s1=1 & s2=0, 3, 'ignoreNull'='false') from root.db.d1
```

Result:
```
+------------------------------------------------------------------------+------------------------------------------------------------------------+
|count_if(root.db.d1.s1 = 0 & root.db.d1.s2 = 0, 3, "ignoreNull"="false")|count_if(root.db.d1.s1 = 1 & root.db.d1.s2 = 0, 3, "ignoreNull"="false")|
+------------------------------------------------------------------------+------------------------------------------------------------------------+
|                                                                       1|                                                                       1|
+------------------------------------------------------------------------+------------------------------------------------------------------------+
```

### TIME_DURATION
#### Grammar
```sql
    time_duration(Path)
```
#### Example
##### raw data
```sql
+----------+-------------+
|      Time|root.db.d1.s1|
+----------+-------------+
|         1|           70|
|         3|           10|
|         4|          303|
|         6|          110|
|         7|          302|
|         8|          110|
|         9|           60|
|        10|           70|
|1677570934|           30|
+----------+-------------+
```
##### Insert sql
```sql
"CREATE DATABASE root.db",
"CREATE TIMESERIES root.db.d1.s1 WITH DATATYPE=INT32, ENCODING=PLAIN tags(city=Beijing)",
"INSERT INTO root.db.d1(timestamp,s1,s2,s3) values(1, 2, 10, true)",
"INSERT INTO root.db.d1(timestamp,s1,s2,s3) values(2, null, 20, true)",
"INSERT INTO root.db.d1(timestamp,s1,s2,s3) values(3, 10, 0, null)",
"INSERT INTO root.db.d1(timestamp,s1,s2,s3) values(4, 303, 30, true)",
"INSERT INTO root.db.d1(timestamp,s1,s2,s3) values(5, null, 20, true)",
"INSERT INTO root.db.d1(timestamp,s1,s2,s3) values(6, 110, 20, true)",
"INSERT INTO root.db.d1(timestamp,s1,s2,s3) values(7, 302, 20, true)",
"INSERT INTO root.db.d1(timestamp,s1,s2,s3) values(8, 110, null, true)",
"INSERT INTO root.db.d1(timestamp,s1,s2,s3) values(9, 60, 20, true)",
"INSERT INTO root.db.d1(timestamp,s1,s2,s3) values(10,70, 20, null)",
"INSERT INTO root.db.d1(timestamp,s1,s2,s3) values(1677570934, 30, 0, true)",
```

SQL:
```sql
select time_duration(s1) from root.db.d1
```

Result:
```
+----------------------------+
|time_duration(root.db.d1.s1)|
+----------------------------+
|                  1677570933|
+----------------------------+
```
> Note: Returns 0 if there is only one data point, or null if the data point is null.

### COUNT_TIME
#### Grammar
```sql
    count_time(*)
```
#### Example
##### raw data
```
+----------+-------------+-------------+-------------+-------------+
|      Time|root.db.d1.s1|root.db.d1.s2|root.db.d2.s1|root.db.d2.s2|
+----------+-------------+-------------+-------------+-------------+
|         0|            0|         null|         null|            0|
|         1|         null|            1|            1|         null|
|         2|         null|            2|            2|         null|
|         4|            4|         null|         null|            4|
|         5|            5|            5|            5|            5|
|         7|         null|            7|            7|         null|
|         8|            8|            8|            8|            8|
|         9|         null|            9|         null|         null|
+----------+-------------+-------------+-------------+-------------+
```
##### Insert sql
```sql
CREATE DATABASE root.db;
CREATE TIMESERIES root.db.d1.s1 WITH DATATYPE=INT32, ENCODING=PLAIN;
CREATE TIMESERIES root.db.d1.s2 WITH DATATYPE=INT32, ENCODING=PLAIN;
CREATE TIMESERIES root.db.d2.s1 WITH DATATYPE=INT32, ENCODING=PLAIN;
CREATE TIMESERIES root.db.d2.s2 WITH DATATYPE=INT32, ENCODING=PLAIN;
INSERT INTO root.db.d1(time, s1) VALUES(0, 0), (4,4), (5,5), (8,8);
INSERT INTO root.db.d1(time, s2) VALUES(1, 1), (2,2), (5,5), (7,7), (8,8), (9,9);
INSERT INTO root.db.d2(time, s1) VALUES(1, 1), (2,2), (5,5), (7,7), (8,8);
INSERT INTO root.db.d2(time, s2) VALUES(0, 0), (4,4), (5,5), (8,8);
```

Query-Example - 1:
```sql
select count_time(*) from root.db.**
```

Result
```
+-------------+
|count_time(*)|
+-------------+
|            8|
+-------------+
```

Query-Example - 2:
```sql
select count_time(*) from root.db.d1, root.db.d2
```

Result
```
+-------------+
|count_time(*)|
+-------------+
|            8|
+-------------+
```

Query-Example - 3:
```sql
select count_time(*) from root.db.** group by([0, 10), 2ms)
```

Result
``` 
+-----------------------------+-------------+
|                         Time|count_time(*)|
+-----------------------------+-------------+
|1970-01-01T08:00:00.000+08:00|            2|            
|1970-01-01T08:00:00.002+08:00|            1|            
|1970-01-01T08:00:00.004+08:00|            2|            
|1970-01-01T08:00:00.006+08:00|            1|            
|1970-01-01T08:00:00.008+08:00|            2|            
+-----------------------------+-------------+
```

Query-Example - 4:
```sql
select count_time(*) from root.db.** group by([0, 10), 2ms) align by device
```

Result
```
+-----------------------------+----------+-------------+
|                         Time|    Device|count_time(*)|
+-----------------------------+----------+-------------+
|1970-01-01T08:00:00.000+08:00|root.db.d1|            2|
|1970-01-01T08:00:00.002+08:00|root.db.d1|            1|
|1970-01-01T08:00:00.004+08:00|root.db.d1|            2|
|1970-01-01T08:00:00.006+08:00|root.db.d1|            1|
|1970-01-01T08:00:00.008+08:00|root.db.d1|            2|
|1970-01-01T08:00:00.000+08:00|root.db.d2|            2|
|1970-01-01T08:00:00.002+08:00|root.db.d2|            1|
|1970-01-01T08:00:00.004+08:00|root.db.d2|            2|
|1970-01-01T08:00:00.006+08:00|root.db.d2|            1|
|1970-01-01T08:00:00.008+08:00|root.db.d2|            1|
+-----------------------------+----------+-------------+
```

> Note:
> 1. The parameter in count_time can only be *.
> 2. Count_time aggregation cannot be used with other aggregation functions.
> 3. Count_time aggregation used with having statement is not supported, and count_time aggregation can not appear in the having statement.
> 4. Count_time does not support use with group by level, group by tag.


### MAX_BY

#### Function Definition
`max_by(x, y)`: Returns the value of x at the timestamp where y reaches its maximum.

- `max_by` must have two input parameters x and y.
- The first input can be the time keyword. `max_by(time, x)` returns the timestamp when x reaches its maximum value.
- Returns null if x is null at the timestamp where y is maximum.
- If y achieves its maximum at multiple timestamps, returns the x value at the smallest timestamp among them.
- Aligns with IoTDB's `max_value`: Only supports INT32, INT64, FLOAT, DOUBLE as y input. Supports all six data types as x input.
- Inputs x and y cannot be literal values.

#### Syntax
```sql
select max_by(x, y) from root.sg
select max_by(time, x) from root.sg
```

#### Examples

##### Raw Data
```sql
IoTDB> select * from root.test
+-----------------------------+-----------+-----------+
|                         Time|root.test.a|root.test.b|
+-----------------------------+-----------+-----------+
|1970-01-01T08:00:00.001+08:00|        1.0|       10.0|
|1970-01-01T08:00:00.002+08:00|        2.0|       10.0|
|1970-01-01T08:00:00.003+08:00|        3.0|        3.0|
|1970-01-01T08:00:00.004+08:00|       10.0|       10.0|
|1970-01-01T08:00:00.005+08:00|       10.0|       12.0|
|1970-01-01T08:00:00.006+08:00|        6.0|        6.0|
+-----------------------------+-----------+-----------+
```
##### Query Examples
Get timestamp corresponding to maximum value: 
```sql
IoTDB> select max_by(time, a), max_value(a) from root.test
+-------------------------+------------------------+
|max_by(Time, root.test.a)|  max_value(root.test.a)|
+-------------------------+------------------------+
|                        4|                    10.0|
+-------------------------+------------------------+
```

Get b value when a is maximum:
```sql
IoTDB> select max_by(b, a) from root.test
+--------------------------------+
|max_by(root.test.b, root.test.a)|
+--------------------------------+
|                            10.0|
+--------------------------------+
```

With expressions:
```sql
IoTDB> select max_by(b + 1, a * 2) from root.test
+----------------------------------------+
|max_by(root.test.b + 1, root.test.a * 2)|
+----------------------------------------+
|                                    11.0|
+----------------------------------------+
```

With GROUP BY TIME clause:
```sql
IoTDB> select max_by(b, a) from root.test group by ([0,7),4ms)
+-----------------------------+--------------------------------+
|                         Time|max_by(root.test.b, root.test.a)|
+-----------------------------+--------------------------------+
|1970-01-01T08:00:00.000+08:00|                             3.0|
+-----------------------------+--------------------------------+
|1970-01-01T08:00:00.004+08:00|                            10.0|
+-----------------------------+--------------------------------+
```

With HAVING clause:
```sql
IoTDB> select max_by(b, a) from root.test group by ([0,7),4ms) having max_by(b, a) > 4.0
+-----------------------------+--------------------------------+
|                         Time|max_by(root.test.b, root.test.a)|
+-----------------------------+--------------------------------+
|1970-01-01T08:00:00.004+08:00|                            10.0|
+-----------------------------+--------------------------------+
```
With ORDER BY clause:
```sql
IoTDB> select max_by(b, a) from root.test group by ([0,7),4ms) order by time desc
+-----------------------------+--------------------------------+
|                         Time|max_by(root.test.b, root.test.a)|
+-----------------------------+--------------------------------+
|1970-01-01T08:00:00.004+08:00|                            10.0|
+-----------------------------+--------------------------------+
|1970-01-01T08:00:00.000+08:00|                             3.0|
+-----------------------------+--------------------------------+
```

### MIN_BY
#### Function Definition
`min_by(x, y)`: Returns the value of x at the timestamp where y reaches its minimum.

- `min_by` must have two input parameters x and y.
- The first input can be the time keyword. `min_by(time, x)` returns the timestamp when x reaches its minimum value.
- Returns null if x is null at the timestamp where y is minimum.
- If y achieves its minimum at multiple timestamps, returns the x value at the smallest timestamp among them.
- Aligns with IoTDB's `min_value`: Only supports INT32, INT64, FLOAT, DOUBLE as y input. Supports all six data types as x input.
- Inputs x and y cannot be literal values.

#### Syntax
```sql
select min_by(x, y) from root.sg
select min_by(time, x) from root.sg
```

#### Examples

##### Raw Data
```sql
IoTDB> select * from root.test
+-----------------------------+-----------+-----------+
|                         Time|root.test.a|root.test.b|
+-----------------------------+-----------+-----------+
|1970-01-01T08:00:00.001+08:00|        4.0|       10.0|
|1970-01-01T08:00:00.002+08:00|        3.0|       10.0|
|1970-01-01T08:00:00.003+08:00|        2.0|        3.0|
|1970-01-01T08:00:00.004+08:00|        1.0|       10.0|
|1970-01-01T08:00:00.005+08:00|        1.0|       12.0|
|1970-01-01T08:00:00.006+08:00|        6.0|        6.0|
+-----------------------------+-----------+-----------+
```
##### Query Examples
Get timestamp corresponding to minimum value:
```sql
IoTDB> select min_by(time, a), min_value(a) from root.test
+-------------------------+------------------------+
|min_by(Time, root.test.a)|  min_value(root.test.a)|
+-------------------------+------------------------+
|                        4|                     1.0|
+-------------------------+------------------------+
```

Get b value when a is minimum:
```sql
IoTDB> select min_by(b, a) from root.test
+--------------------------------+
|min_by(root.test.b, root.test.a)|
+--------------------------------+
|                            10.0|
+--------------------------------+
```

With expressions:
```sql
IoTDB> select min_by(b + 1, a * 2) from root.test
+----------------------------------------+
|min_by(root.test.b + 1, root.test.a * 2)|
+----------------------------------------+
|                                    11.0|
+----------------------------------------+
```

With GROUP BY TIME clause:
```sql
IoTDB> select min_by(b, a) from root.test group by ([0,7),4ms)
+-----------------------------+--------------------------------+
|                         Time|min_by(root.test.b, root.test.a)|
+-----------------------------+--------------------------------+
|1970-01-01T08:00:00.000+08:00|                             3.0|
+-----------------------------+--------------------------------+
|1970-01-01T08:00:00.004+08:00|                            10.0|
+-----------------------------+--------------------------------+
```

With HAVING clause:
```sql
IoTDB> select min_by(b, a) from root.test group by ([0,7),4ms) having max_by(b, a) > 4.0
+-----------------------------+--------------------------------+
|                         Time|min_by(root.test.b, root.test.a)|
+-----------------------------+--------------------------------+
|1970-01-01T08:00:00.004+08:00|                            10.0|
+-----------------------------+--------------------------------+
```

With ORDER BY clause:
```sql
IoTDB> select min_by(b, a) from root.test group by ([0,7),4ms) order by time desc
+-----------------------------+--------------------------------+
|                         Time|min_by(root.test.b, root.test.a)|
+-----------------------------+--------------------------------+
|1970-01-01T08:00:00.004+08:00|                            10.0|
+-----------------------------+--------------------------------+
|1970-01-01T08:00:00.000+08:00|                             3.0|
+-----------------------------+--------------------------------+
```


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## String Processing

### STRING_CONTAINS

#### Function introduction

This function checks whether the substring `s` exists in the string

**Function name:** STRING_CONTAINS

**Input sequence:** Only a single input sequence is supported, the type is TEXT.

**parameter:**
+ `s`: The string to search for.

**Output Sequence:** Output a single sequence, the type is BOOLEAN.

#### Usage example

```   sql
select s1, string_contains(s1, 's'='warn') from root.sg1.d4;
```

``` 
+-----------------------------+--------------+-------------------------------------------+
|                         Time|root.sg1.d4.s1|string_contains(root.sg1.d4.s1, "s"="warn")|
+-----------------------------+--------------+-------------------------------------------+
|1970-01-01T08:00:00.001+08:00|    warn:-8721|                                       true|
|1970-01-01T08:00:00.002+08:00|  error:-37229|                                      false|
|1970-01-01T08:00:00.003+08:00|     warn:1731|                                       true|
+-----------------------------+--------------+-------------------------------------------+
Total line number = 3
It costs 0.007s
```

### STRING_MATCHES

#### Function introduction

This function judges whether a string can be matched by the regular expression `regex`.

**Function name:** STRING_MATCHES

**Input sequence:** Only a single input sequence is supported, the type is TEXT.

**parameter:**
+ `regex`: Java standard library-style regular expressions.

**Output Sequence:** Output a single sequence, the type is BOOLEAN.

#### Usage example

```sql
select s1, string_matches(s1, 'regex'='[^\\s]+37229') from root.sg1.d4;
```

```
+-----------------------------+--------------+------------------------------------------------------+
|                         Time|root.sg1.d4.s1|string_matches(root.sg1.d4.s1, "regex"="[^\\s]+37229")|
+-----------------------------+--------------+------------------------------------------------------+
|1970-01-01T08:00:00.001+08:00|    warn:-8721|                                                 false|
|1970-01-01T08:00:00.002+08:00|  error:-37229|                                                  true|
|1970-01-01T08:00:00.003+08:00|     warn:1731|                                                 false|
+-----------------------------+--------------+------------------------------------------------------+
Total line number = 3
It costs 0.007s
```

### Length

#### Usage

The function is used to get the length of input series.

**Name:** LENGTH

**Input Series:** Only support a single input series. The data type is TEXT.

**Output Series:** Output a single series. The type is INT32.

**Note:** Returns NULL if input is NULL.

#### Examples

Input series:

```
+-----------------------------+--------------+
|                         Time|root.sg1.d1.s1|
+-----------------------------+--------------+
|1970-01-01T08:00:00.001+08:00|        1test1|
|1970-01-01T08:00:00.002+08:00|      22test22|
+-----------------------------+--------------+
```

SQL for query:

```sql
select s1, length(s1) from root.sg1.d1
```

Output series:

```
+-----------------------------+--------------+----------------------+
|                         Time|root.sg1.d1.s1|length(root.sg1.d1.s1)|
+-----------------------------+--------------+----------------------+
|1970-01-01T08:00:00.001+08:00|        1test1|                     6|
|1970-01-01T08:00:00.002+08:00|      22test22|                     8|
+-----------------------------+--------------+----------------------+
```

### Locate

#### Usage

The function is used to get the position of the first occurrence of substring `target` in input series. Returns -1 if there are no `target` in input.

**Name:** LOCATE

**Input Series:** Only support a single input series. The data type is TEXT.

**Parameter:**

+ `target`: The substring to be located.
+ `reverse`: Indicates whether reverse locate is required. The default value is `false`, means left-to-right locate.

**Output Series:** Output a single series. The type is INT32.

**Note:** The index begins from 0.

#### Examples

Input series:

```
+-----------------------------+--------------+
|                         Time|root.sg1.d1.s1|
+-----------------------------+--------------+
|1970-01-01T08:00:00.001+08:00|        1test1|
|1970-01-01T08:00:00.002+08:00|      22test22|
+-----------------------------+--------------+
```

SQL for query:

```sql
select s1, locate(s1, "target"="1") from root.sg1.d1
```

Output series:

```
+-----------------------------+--------------+------------------------------------+
|                         Time|root.sg1.d1.s1|locate(root.sg1.d1.s1, "target"="1")|
+-----------------------------+--------------+------------------------------------+
|1970-01-01T08:00:00.001+08:00|        1test1|                                   0|
|1970-01-01T08:00:00.002+08:00|      22test22|                                  -1|
+-----------------------------+--------------+------------------------------------+
```

Another SQL for query:

```sql
select s1, locate(s1, "target"="1", "reverse"="true") from root.sg1.d1
```

Output series:

```
+-----------------------------+--------------+------------------------------------------------------+
|                         Time|root.sg1.d1.s1|locate(root.sg1.d1.s1, "target"="1", "reverse"="true")|
+-----------------------------+--------------+------------------------------------------------------+
|1970-01-01T08:00:00.001+08:00|        1test1|                                                     5|
|1970-01-01T08:00:00.002+08:00|      22test22|                                                    -1|
+-----------------------------+--------------+------------------------------------------------------+
```

### StartsWith

#### Usage

The function is used to check whether input series starts with the specified prefix.

**Name:** STARTSWITH

**Input Series:** Only support a single input series. The data type is TEXT.

**Parameter:**
+ `target`: The prefix to be checked.

**Output Series:** Output a single series. The type is BOOLEAN.

**Note:** Returns NULL if input is NULL.

#### Examples

Input series:

```
+-----------------------------+--------------+
|                         Time|root.sg1.d1.s1|
+-----------------------------+--------------+
|1970-01-01T08:00:00.001+08:00|        1test1|
|1970-01-01T08:00:00.002+08:00|      22test22|
+-----------------------------+--------------+
```

SQL for query:

```sql
select s1, startswith(s1, "target"="1") from root.sg1.d1
```

Output series:

```
+-----------------------------+--------------+----------------------------------------+
|                         Time|root.sg1.d1.s1|startswith(root.sg1.d1.s1, "target"="1")|
+-----------------------------+--------------+----------------------------------------+
|1970-01-01T08:00:00.001+08:00|        1test1|                                    true|
|1970-01-01T08:00:00.002+08:00|      22test22|                                   false|
+-----------------------------+--------------+----------------------------------------+
```

### EndsWith

#### Usage

The function is used to check whether input series ends with the specified suffix.

**Name:** ENDSWITH

**Input Series:** Only support a single input series. The data type is TEXT.

**Parameter:**
+ `target`: The suffix to be checked.

**Output Series:** Output a single series. The type is BOOLEAN.

**Note:** Returns NULL if input is NULL.

#### Examples

Input series:

```
+-----------------------------+--------------+
|                         Time|root.sg1.d1.s1|
+-----------------------------+--------------+
|1970-01-01T08:00:00.001+08:00|        1test1|
|1970-01-01T08:00:00.002+08:00|      22test22|
+-----------------------------+--------------+
```

SQL for query:

```sql
select s1, endswith(s1, "target"="1") from root.sg1.d1
```

Output series:

```
+-----------------------------+--------------+--------------------------------------+
|                         Time|root.sg1.d1.s1|endswith(root.sg1.d1.s1, "target"="1")|
+-----------------------------+--------------+--------------------------------------+
|1970-01-01T08:00:00.001+08:00|        1test1|                                  true|
|1970-01-01T08:00:00.002+08:00|      22test22|                                 false|
+-----------------------------+--------------+--------------------------------------+
```

### Concat

#### Usage

The function is used to concat input series and target strings.

**Name:** CONCAT

**Input Series:** At least one input series. The data type is TEXT.

**Parameter:**
+ `targets`: A series of K-V, key needs to start with `target` and be not duplicated, value is the string you want to concat.
+ `series_behind`: Indicates whether series behind targets. The default value is `false`.

**Output Series:** Output a single series. The type is TEXT.

**Note:** 
+ If value of input series is NULL, it will be skipped.
+ We can only concat input series and `targets` separately. `concat(s1, "target1"="IoT", s2, "target2"="DB")` and
  `concat(s1, s2, "target1"="IoT", "target2"="DB")` gives the same result.

#### Examples

Input series:

```
+-----------------------------+--------------+--------------+
|                         Time|root.sg1.d1.s1|root.sg1.d1.s2|
+-----------------------------+--------------+--------------+
|1970-01-01T08:00:00.001+08:00|        1test1|          null|
|1970-01-01T08:00:00.002+08:00|      22test22|      2222test|
+-----------------------------+--------------+--------------+
```

SQL for query:

```sql
select s1, s2, concat(s1, s2, "target1"="IoT", "target2"="DB") from root.sg1.d1
```

Output series:

```
+-----------------------------+--------------+--------------+-----------------------------------------------------------------------+
|                         Time|root.sg1.d1.s1|root.sg1.d1.s2|concat(root.sg1.d1.s1, root.sg1.d1.s2, "target1"="IoT", "target2"="DB")|
+-----------------------------+--------------+--------------+-----------------------------------------------------------------------+
|1970-01-01T08:00:00.001+08:00|        1test1|          null|                                                            1test1IoTDB|
|1970-01-01T08:00:00.002+08:00|      22test22|      2222test|                                                  22test222222testIoTDB|
+-----------------------------+--------------+--------------+-----------------------------------------------------------------------+
```

Another SQL for query:

```sql
select s1, s2, concat(s1, s2, "target1"="IoT", "target2"="DB", "series_behind"="true") from root.sg1.d1
```

Output series:

```
+-----------------------------+--------------+--------------+-----------------------------------------------------------------------------------------------+
|                         Time|root.sg1.d1.s1|root.sg1.d1.s2|concat(root.sg1.d1.s1, root.sg1.d1.s2, "target1"="IoT", "target2"="DB", "series_behind"="true")|
+-----------------------------+--------------+--------------+-----------------------------------------------------------------------------------------------+
|1970-01-01T08:00:00.001+08:00|        1test1|          null|                                                                                    IoTDB1test1|
|1970-01-01T08:00:00.002+08:00|      22test22|      2222test|                                                                          IoTDB22test222222test|
+-----------------------------+--------------+--------------+-----------------------------------------------------------------------------------------------+
```

### substring

#### Usage

Extracts a substring of a string, starting with the first specified character and stopping after the specified number of characters.The index start at 1. The value range of from and for is an INT32.

**Name:** SUBSTRING

**Input Series:** Only support a single input series. The data type is TEXT.

**Parameter:**
+ `from`: Indicates the start position of substring.
+ `for`: Indicates how many characters to stop after of substring.

**Output Series:** Output a single series. The type is TEXT.

**Note:** Returns NULL if input is NULL.

#### Examples

Input series:

```
+-----------------------------+--------------+
|                         Time|root.sg1.d1.s1|
+-----------------------------+--------------+
|1970-01-01T08:00:00.001+08:00|        1test1|
|1970-01-01T08:00:00.002+08:00|      22test22|
+-----------------------------+--------------+
```

SQL for query:

```sql
select s1, substring(s1 from 1 for 2) from root.sg1.d1
```

Output series:

```
+-----------------------------+--------------+--------------------------------------+
|                         Time|root.sg1.d1.s1|SUBSTRING(root.sg1.d1.s1 FROM 1 FOR 2)|
+-----------------------------+--------------+--------------------------------------+
|1970-01-01T08:00:00.001+08:00|        1test1|                                    1t|
|1970-01-01T08:00:00.002+08:00|      22test22|                                    22|
+-----------------------------+--------------+--------------------------------------+
```

### replace

#### Usage

Replace a substring in the input sequence with the target substring.

**Name:** REPLACE

**Input Series:** Only support a single input series. The data type is TEXT.

**Parameter:**
+ first parameter: The target substring to be replaced.
+ second parameter: The substring to replace with.

**Output Series:** Output a single series. The type is TEXT.

**Note:** Returns NULL if input is NULL.

#### Examples

Input series:

```
+-----------------------------+--------------+
|                         Time|root.sg1.d1.s1|
+-----------------------------+--------------+
|1970-01-01T08:00:00.001+08:00|        1test1|
|1970-01-01T08:00:00.002+08:00|      22test22|
+-----------------------------+--------------+
```

SQL for query:

```sql
select s1, replace(s1, 'es', 'tt') from root.sg1.d1
```

Output series:

```
+-----------------------------+--------------+-----------------------------------+
|                         Time|root.sg1.d1.s1|REPLACE(root.sg1.d1.s1, 'es', 'tt')|
+-----------------------------+--------------+-----------------------------------+
|1970-01-01T08:00:00.001+08:00|        1test1|                             1tttt1|
|1970-01-01T08:00:00.002+08:00|      22test22|                           22tttt22|
+-----------------------------+--------------+-----------------------------------+
```

### Upper

#### Usage

The function is used to get the string of input series with all characters changed to uppercase.

**Name:** UPPER

**Input Series:** Only support a single input series. The data type is TEXT.

**Output Series:** Output a single series. The type is TEXT.

**Note:** Returns NULL if input is NULL.

#### Examples

Input series:

```
+-----------------------------+--------------+
|                         Time|root.sg1.d1.s1|
+-----------------------------+--------------+
|1970-01-01T08:00:00.001+08:00|        1test1|
|1970-01-01T08:00:00.002+08:00|      22test22|
+-----------------------------+--------------+
```

SQL for query:

```sql
select s1, upper(s1) from root.sg1.d1
```

Output series:

```
+-----------------------------+--------------+---------------------+
|                         Time|root.sg1.d1.s1|upper(root.sg1.d1.s1)|
+-----------------------------+--------------+---------------------+
|1970-01-01T08:00:00.001+08:00|        1test1|               1TEST1|
|1970-01-01T08:00:00.002+08:00|      22test22|             22TEST22|
+-----------------------------+--------------+---------------------+
```

### Lower

#### Usage

The function is used to get the string of input series with all characters changed to lowercase.

**Name:** LOWER

**Input Series:** Only support a single input series. The data type is TEXT.

**Output Series:** Output a single series. The type is TEXT.

**Note:** Returns NULL if input is NULL.

#### Examples

Input series:

```
+-----------------------------+--------------+
|                         Time|root.sg1.d1.s1|
+-----------------------------+--------------+
|1970-01-01T08:00:00.001+08:00|        1TEST1|
|1970-01-01T08:00:00.002+08:00|      22TEST22|
+-----------------------------+--------------+
```

SQL for query:

```sql
select s1, lower(s1) from root.sg1.d1
```

Output series:

```
+-----------------------------+--------------+---------------------+
|                         Time|root.sg1.d1.s1|lower(root.sg1.d1.s1)|
+-----------------------------+--------------+---------------------+
|1970-01-01T08:00:00.001+08:00|        1TEST1|               1test1|
|1970-01-01T08:00:00.002+08:00|      22TEST22|             22test22|
+-----------------------------+--------------+---------------------+
```

### Trim

#### Usage

The function is used to get the string whose value is same to input series, with all leading and trailing space removed.

**Name:** TRIM

**Input Series:** Only support a single input series. The data type is TEXT.

**Output Series:** Output a single series. The type is TEXT.

**Note:** Returns NULL if input is NULL.

#### Examples

Input series:

```
+-----------------------------+--------------+
|                         Time|root.sg1.d1.s3|
+-----------------------------+--------------+
|1970-01-01T08:00:00.002+08:00|   3querytest3|
|1970-01-01T08:00:00.003+08:00|  3querytest3 |
+-----------------------------+--------------+
```

SQL for query:

```sql
select s3, trim(s3) from root.sg1.d1
```

Output series:

```
+-----------------------------+--------------+--------------------+
|                         Time|root.sg1.d1.s3|trim(root.sg1.d1.s3)|
+-----------------------------+--------------+--------------------+
|1970-01-01T08:00:00.002+08:00|   3querytest3|         3querytest3|
|1970-01-01T08:00:00.003+08:00|  3querytest3 |         3querytest3|
+-----------------------------+--------------+--------------------+
```

### StrCmp

#### Usage

The function is used to get the compare result of two input series. Returns `0` if series value are the same, a `negative integer` if value of series1 is smaller than series2, 
a `positive integer` if value of series1  is more than series2.

**Name:** StrCmp

**Input Series:** Support two input series. Data types are all the TEXT.

**Output Series:** Output a single series. The type is INT32.

**Note:** Returns NULL either series value is NULL.

#### Examples

Input series:

```
+-----------------------------+--------------+--------------+
|                         Time|root.sg1.d1.s1|root.sg1.d1.s2|
+-----------------------------+--------------+--------------+
|1970-01-01T08:00:00.001+08:00|        1test1|          null|
|1970-01-01T08:00:00.002+08:00|      22test22|      2222test|
+-----------------------------+--------------+--------------+
```

SQL for query:

```sql
select s1, s2, strcmp(s1, s2) from root.sg1.d1
```

Output series:

```
+-----------------------------+--------------+--------------+--------------------------------------+
|                         Time|root.sg1.d1.s1|root.sg1.d1.s2|strcmp(root.sg1.d1.s1, root.sg1.d1.s2)|
+-----------------------------+--------------+--------------+--------------------------------------+
|1970-01-01T08:00:00.001+08:00|        1test1|          null|                                  null|
|1970-01-01T08:00:00.002+08:00|      22test22|      2222test|                                    66|
+-----------------------------+--------------+--------------+--------------------------------------+
```


### StrReplace

#### Usage

**This is not a built-in function and can only be used after registering the library-udf.** The function is used to replace the specific substring with given string.

**Name:** STRREPLACE

**Input Series:** Only support a single input series. The data type is TEXT.

**Parameter:**

+ `target`: The target substring to be replaced.
+ `replace`: The string to be put on.
+ `limit`: The number of matches to be replaced which should be an integer no less than -1,
  default to -1 which means all matches will be replaced.
+ `offset`: The number of matches to be skipped, which means the first `offset` matches will not be replaced, default to 0.
+ `reverse`: Whether to count all the matches reversely, default to 'false'.

**Output Series:** Output a single series. The type is TEXT.

#### Examples

Input series:

```
+-----------------------------+---------------+
|                         Time|root.test.d1.s1|
+-----------------------------+---------------+
|2021-01-01T00:00:01.000+08:00|      A,B,A+,B-|
|2021-01-01T00:00:02.000+08:00|      A,A+,A,B+|
|2021-01-01T00:00:03.000+08:00|         B+,B,B|
|2021-01-01T00:00:04.000+08:00|      A+,A,A+,A|
|2021-01-01T00:00:05.000+08:00|       A,B-,B,B|
+-----------------------------+---------------+
```

SQL for query:

```sql
select strreplace(s1, "target"=",", "replace"="/", "limit"="2") from root.test.d1
```

Output series:

```
+-----------------------------+-----------------------------------------+
|                         Time|strreplace(root.test.d1.s1, "target"=",",|
|                             |              "replace"="/", "limit"="2")|
+-----------------------------+-----------------------------------------+
|2021-01-01T00:00:01.000+08:00|                                A/B/A+,B-|
|2021-01-01T00:00:02.000+08:00|                                A/A+/A,B+|
|2021-01-01T00:00:03.000+08:00|                                   B+/B/B|
|2021-01-01T00:00:04.000+08:00|                                A+/A/A+,A|
|2021-01-01T00:00:05.000+08:00|                                 A/B-/B,B|
+-----------------------------+-----------------------------------------+
```

Another SQL for query:

```sql
select strreplace(s1, "target"=",", "replace"="/", "limit"="1", "offset"="1", "reverse"="true") from root.test.d1
```

Output series:

```
+-----------------------------+-----------------------------------------------------+
|                         Time|strreplace(root.test.d1.s1, "target"=",", "replace"= | 
|                             |    "|", "limit"="1", "offset"="1", "reverse"="true")|
+-----------------------------+-----------------------------------------------------+
|2021-01-01T00:00:01.000+08:00|                                            A,B/A+,B-|
|2021-01-01T00:00:02.000+08:00|                                            A,A+/A,B+|
|2021-01-01T00:00:03.000+08:00|                                               B+/B,B|
|2021-01-01T00:00:04.000+08:00|                                            A+,A/A+,A|
|2021-01-01T00:00:05.000+08:00|                                             A,B-/B,B|
+-----------------------------+-----------------------------------------------------+
```

### RegexMatch

#### Usage

**This is not a built-in function and can only be used after registering the library-udf.** The function is used to fetch matched contents from text with given regular expression.

**Name:** REGEXMATCH

**Input Series:** Only support a single input series. The data type is TEXT.

**Parameter:**

+ `regex`: The regular expression to match in the text. All grammars supported by Java are acceptable,
  for example, `\d+\.\d+\.\d+\.\d+` is expected to match any IPv4 addresses.
+ `group`: The wanted group index in the matched result.
  Reference to java.util.regex, group 0 is the whole pattern and
  the next ones are numbered with the appearance order of left parentheses.
  For example, the groups in `A(B(CD))` are: 0-`A(B(CD))`, 1-`B(CD)`, 2-`CD`.

**Output Series:** Output a single series. The type is TEXT.

**Note:** Those points with null values or not matched with the given pattern will not return any results.

#### Examples

Input series:

```
+-----------------------------+-------------------------------+
|                         Time|                root.test.d1.s1|
+-----------------------------+-------------------------------+
|2021-01-01T00:00:01.000+08:00|        [192.168.0.1] [SUCCESS]|
|2021-01-01T00:00:02.000+08:00|       [192.168.0.24] [SUCCESS]|
|2021-01-01T00:00:03.000+08:00|           [192.168.0.2] [FAIL]|
|2021-01-01T00:00:04.000+08:00|        [192.168.0.5] [SUCCESS]|
|2021-01-01T00:00:05.000+08:00|      [192.168.0.124] [SUCCESS]|
+-----------------------------+-------------------------------+
```

SQL for query:

```sql
select regexmatch(s1, "regex"="\d+\.\d+\.\d+\.\d+", "group"="0") from root.test.d1
```

Output series:

```
+-----------------------------+----------------------------------------------------------------------+
|                         Time|regexmatch(root.test.d1.s1, "regex"="\d+\.\d+\.\d+\.\d+", "group"="0")|
+-----------------------------+----------------------------------------------------------------------+
|2021-01-01T00:00:01.000+08:00|                                                           192.168.0.1|
|2021-01-01T00:00:02.000+08:00|                                                          192.168.0.24|
|2021-01-01T00:00:03.000+08:00|                                                           192.168.0.2|
|2021-01-01T00:00:04.000+08:00|                                                           192.168.0.5|
|2021-01-01T00:00:05.000+08:00|                                                         192.168.0.124|
+-----------------------------+----------------------------------------------------------------------+
```

### RegexReplace

#### Usage

**This is not a built-in function and can only be used after registering the library-udf.** The function is used to replace the specific regular expression matches with given string.

**Name:** REGEXREPLACE

**Input Series:** Only support a single input series. The data type is TEXT.

**Parameter:**

+ `regex`: The target regular expression to be replaced. All grammars supported by Java are acceptable.
+ `replace`: The string to be put on and back reference notes in Java is also supported,
  for example, '$1' refers to group 1 in the `regex` which will be filled with corresponding matched results.
+ `limit`: The number of matches to be replaced which should be an integer no less than -1,
  default to -1 which means all matches will be replaced.
+ `offset`: The number of matches to be skipped, which means the first `offset` matches will not be replaced, default to 0.
+ `reverse`: Whether to count all the matches reversely, default to 'false'.

**Output Series:** Output a single series. The type is TEXT.

#### Examples

Input series:

```
+-----------------------------+-------------------------------+
|                         Time|                root.test.d1.s1|
+-----------------------------+-------------------------------+
|2021-01-01T00:00:01.000+08:00|        [192.168.0.1] [SUCCESS]|
|2021-01-01T00:00:02.000+08:00|       [192.168.0.24] [SUCCESS]|
|2021-01-01T00:00:03.000+08:00|           [192.168.0.2] [FAIL]|
|2021-01-01T00:00:04.000+08:00|        [192.168.0.5] [SUCCESS]|
|2021-01-01T00:00:05.000+08:00|      [192.168.0.124] [SUCCESS]|
+-----------------------------+-------------------------------+
```

SQL for query:

```sql
select regexreplace(s1, "regex"="192\.168\.0\.(\d+)", "replace"="cluster-$1", "limit"="1") from root.test.d1
```

Output series:

```
+-----------------------------+-----------------------------------------------------------+
|                         Time|regexreplace(root.test.d1.s1, "regex"="192\.168\.0\.(\d+)",|
|                             |                       "replace"="cluster-$1", "limit"="1")|
+-----------------------------+-----------------------------------------------------------+
|2021-01-01T00:00:01.000+08:00|                                      [cluster-1] [SUCCESS]|
|2021-01-01T00:00:02.000+08:00|                                     [cluster-24] [SUCCESS]|
|2021-01-01T00:00:03.000+08:00|                                         [cluster-2] [FAIL]|
|2021-01-01T00:00:04.000+08:00|                                      [cluster-5] [SUCCESS]|
|2021-01-01T00:00:05.000+08:00|                                    [cluster-124] [SUCCESS]|
+-----------------------------+-----------------------------------------------------------+
```

### RegexSplit

#### Usage

**This is not a built-in function and can only be used after registering the library-udf.** The function is used to split text with given regular expression and return specific element.

**Name:** REGEXSPLIT

**Input Series:** Only support a single input series. The data type is TEXT.

**Parameter:**

+ `regex`: The regular expression used to split the text.
  All grammars supported by Java are acceptable, for example, `['"]` is expected to match `'` and `"`.
+ `index`: The wanted index of elements in the split result.
  It should be an integer no less than -1, default to -1 which means the length of the result array is returned
  and any non-negative integer is used to fetch the text of the specific index starting from 0.

**Output Series:** Output a single series. The type is INT32 when `index` is -1 and TEXT when it's an valid index.

**Note:** When `index` is out of the range of the result array, for example `0,1,2` split with `,` and `index` is set to 3,
no result are returned for that record.

#### Examples

Input series:

```
+-----------------------------+---------------+
|                         Time|root.test.d1.s1|
+-----------------------------+---------------+
|2021-01-01T00:00:01.000+08:00|      A,B,A+,B-|
|2021-01-01T00:00:02.000+08:00|      A,A+,A,B+|
|2021-01-01T00:00:03.000+08:00|         B+,B,B|
|2021-01-01T00:00:04.000+08:00|      A+,A,A+,A|
|2021-01-01T00:00:05.000+08:00|       A,B-,B,B|
+-----------------------------+---------------+
```

SQL for query:

```sql
select regexsplit(s1, "regex"=",", "index"="-1") from root.test.d1
```

Output series:

```
+-----------------------------+------------------------------------------------------+
|                         Time|regexsplit(root.test.d1.s1, "regex"=",", "index"="-1")|
+-----------------------------+------------------------------------------------------+
|2021-01-01T00:00:01.000+08:00|                                                     4|
|2021-01-01T00:00:02.000+08:00|                                                     4|
|2021-01-01T00:00:03.000+08:00|                                                     3|
|2021-01-01T00:00:04.000+08:00|                                                     4|
|2021-01-01T00:00:05.000+08:00|                                                     4|
+-----------------------------+------------------------------------------------------+
```

Another SQL for query:

SQL for query:

```sql
select regexsplit(s1, "regex"=",", "index"="3") from root.test.d1
```

Output series:

```
+-----------------------------+-----------------------------------------------------+
|                         Time|regexsplit(root.test.d1.s1, "regex"=",", "index"="3")|
+-----------------------------+-----------------------------------------------------+
|2021-01-01T00:00:01.000+08:00|                                                   B-|
|2021-01-01T00:00:02.000+08:00|                                                   B+|
|2021-01-01T00:00:04.000+08:00|                                                    A|
|2021-01-01T00:00:05.000+08:00|                                                    B|
+-----------------------------+-----------------------------------------------------+
```

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## Data Type Conversion Function

The IoTDB currently supports 6 data types, including INT32, INT64 ,FLOAT, DOUBLE, BOOLEAN, TEXT. When we query or evaluate data, we may need to convert data types, such as TEXT to INT32, or FLOAT to DOUBLE. IoTDB supports cast function to convert data types.

Syntax example:

```sql
SELECT cast(s1 as INT32) from root.sg
```

The syntax of the cast function is consistent with that of PostgreSQL. The data type specified after AS indicates the target type to be converted. Currently, all six data types supported by IoTDB can be used in the cast function. The conversion rules to be followed are shown in the following table. The row represents the original data type, and the column represents the target data type to be converted into:

|             | **INT32**                                                    | **INT64**                                                    | **FLOAT**                                                    | **DOUBLE**              | **BOOLEAN**                                                  | **TEXT**                         |
| ----------- | ------------------------------------------------------------ | ------------------------------------------------------------ | ------------------------------------------------------------ | ----------------------- | ------------------------------------------------------------ | -------------------------------- |
| **INT32**   | No need to cast                                              | Cast directly                                                | Cast directly                                                | Cast directly           | !=0 : true<br />==0: false                                   | String.valueOf()                 |
| **INT64**   | Out of the range of INT32: throw Exception<br />Otherwise: Cast directly | No need to cast                                              | Cast directly                                                | Cast directly           | !=0L : true<br />==0: false                                  | String.valueOf()                 |
| **FLOAT**   | Out of the range of INT32: throw Exception<br />Otherwise: Math.round() | Out of the range of INT64: throw Exception<br />Otherwise: Math.round() | No need to cast                                              | Cast directly           | !=0.0f : true<br />==0: false                                | String.valueOf()                 |
| **DOUBLE**  | Out of the range of INT32: throw Exception<br />Otherwise: Math.round() | Out of the range of INT64: throw Exception<br />Otherwise: Math.round() | Out of the range of FLOAT：throw Exception<br />Otherwise: Cast directly | No need to cast         | !=0.0 : true<br />==0: false                                 | String.valueOf()                 |
| **BOOLEAN** | true: 1<br />false: 0                                        | true: 1L<br />false: 0                                       | true: 1.0f<br />false: 0                                     | true: 1.0<br />false: 0 | No need to cast                                              | true: "true"<br />false: "false" |
| **TEXT**    | Integer.parseInt()                                           | Long.parseLong()                                             | Float.parseFloat()                                           | Double.parseDouble()    | text.toLowerCase =="true" : true<br />text.toLowerCase =="false" : false<br />Otherwise: throw Exception | No need to cast                  |

### Examples

```
// timeseries
IoTDB> show timeseries root.sg.d1.**
+-------------+-----+--------+--------+--------+-----------+----+----------+--------+------------------+
|   Timeseries|Alias|Database|DataType|Encoding|Compression|Tags|Attributes|Deadband|DeadbandParameters|
+-------------+-----+--------+--------+--------+-----------+----+----------+--------+------------------+
|root.sg.d1.s3| null| root.sg|   FLOAT|   PLAIN|     SNAPPY|null|      null|    null|              null|
|root.sg.d1.s4| null| root.sg|  DOUBLE|   PLAIN|     SNAPPY|null|      null|    null|              null|
|root.sg.d1.s5| null| root.sg| BOOLEAN|   PLAIN|     SNAPPY|null|      null|    null|              null|
|root.sg.d1.s6| null| root.sg|    TEXT|   PLAIN|     SNAPPY|null|      null|    null|              null|
|root.sg.d1.s1| null| root.sg|   INT32|   PLAIN|     SNAPPY|null|      null|    null|              null|
|root.sg.d1.s2| null| root.sg|   INT64|   PLAIN|     SNAPPY|null|      null|    null|              null|
+-------------+-----+--------+--------+--------+-----------+----+----------+--------+------------------+

// data of timeseries
IoTDB> select * from root.sg.d1;
+-----------------------------+-------------+-------------+-------------+-------------+-------------+-------------+
|                         Time|root.sg.d1.s3|root.sg.d1.s4|root.sg.d1.s5|root.sg.d1.s6|root.sg.d1.s1|root.sg.d1.s2|
+-----------------------------+-------------+-------------+-------------+-------------+-------------+-------------+
|1970-01-01T08:00:00.000+08:00|          0.0|          0.0|        false|        10000|            0|            0|
|1970-01-01T08:00:00.001+08:00|          1.0|          1.0|        false|            3|            1|            1|
|1970-01-01T08:00:00.002+08:00|          2.7|          2.7|         true|         TRue|            2|            2|
|1970-01-01T08:00:00.003+08:00|         3.33|         3.33|         true|        faLse|            3|            3|
+-----------------------------+-------------+-------------+-------------+-------------+-------------+-------------+

// cast BOOLEAN to other types
IoTDB> select cast(s5 as INT32), cast(s5 as INT64),cast(s5 as FLOAT),cast(s5 as DOUBLE), cast(s5 as TEXT) from root.sg.d1
+-----------------------------+----------------------------+----------------------------+----------------------------+-----------------------------+---------------------------+
|                         Time|CAST(root.sg.d1.s5 AS INT32)|CAST(root.sg.d1.s5 AS INT64)|CAST(root.sg.d1.s5 AS FLOAT)|CAST(root.sg.d1.s5 AS DOUBLE)|CAST(root.sg.d1.s5 AS TEXT)|
+-----------------------------+----------------------------+----------------------------+----------------------------+-----------------------------+---------------------------+
|1970-01-01T08:00:00.000+08:00|                           0|                           0|                         0.0|                          0.0|                      false|
|1970-01-01T08:00:00.001+08:00|                           0|                           0|                         0.0|                          0.0|                      false|
|1970-01-01T08:00:00.002+08:00|                           1|                           1|                         1.0|                          1.0|                       true|
|1970-01-01T08:00:00.003+08:00|                           1|                           1|                         1.0|                          1.0|                       true|
+-----------------------------+----------------------------+----------------------------+----------------------------+-----------------------------+---------------------------+

// cast TEXT to numeric types
IoTDB> select cast(s6 as INT32), cast(s6 as INT64), cast(s6 as FLOAT), cast(s6 as DOUBLE) from root.sg.d1 where time < 2
+-----------------------------+----------------------------+----------------------------+----------------------------+-----------------------------+
|                         Time|CAST(root.sg.d1.s6 AS INT32)|CAST(root.sg.d1.s6 AS INT64)|CAST(root.sg.d1.s6 AS FLOAT)|CAST(root.sg.d1.s6 AS DOUBLE)|
+-----------------------------+----------------------------+----------------------------+----------------------------+-----------------------------+
|1970-01-01T08:00:00.000+08:00|                       10000|                       10000|                     10000.0|                      10000.0|
|1970-01-01T08:00:00.001+08:00|                           3|                           3|                         3.0|                          3.0|
+-----------------------------+----------------------------+----------------------------+----------------------------+-----------------------------+

// cast TEXT to BOOLEAN
IoTDB> select cast(s6 as BOOLEAN) from root.sg.d1 where time >= 2
+-----------------------------+------------------------------+
|                         Time|CAST(root.sg.d1.s6 AS BOOLEAN)|
+-----------------------------+------------------------------+
|1970-01-01T08:00:00.002+08:00|                          true|
|1970-01-01T08:00:00.003+08:00|                         false|
+-----------------------------+------------------------------+
```


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    or more contributor license agreements.  See the NOTICE file
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    "License"); you may not use this file except in compliance
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## Constant Timeseries Generating Functions

The constant timeseries generating function is used to generate a timeseries in which the values of all data points are the same.

The constant timeseries generating function accepts one or more timeseries inputs, and the timestamp set of the output data points is the union of the timestamp sets of the input timeseries.

Currently, IoTDB supports the following constant timeseries generating functions:

| Function Name | Required Attributes                                          | Output Series Data Type                      | Description                                                  |
| ------------- | ------------------------------------------------------------ | -------------------------------------------- | ------------------------------------------------------------ |
| CONST         | `value`: the value of the output data point <br />`type`: the type of the output data point, it can only be INT32 / INT64 / FLOAT / DOUBLE / BOOLEAN / TEXT | Determined by the required attribute  `type` | Output the user-specified constant timeseries according to the  attributes `value` and `type`. |
| PI            | None                                                         | DOUBLE                                       | Data point value: a `double` value of  `π`, the ratio of the circumference of a circle to its diameter, which is equals to `Math.PI` in the *Java Standard Library*. |
| E             | None                                                         | DOUBLE                                       | Data point value: a `double` value of  `e`, the base of the natural logarithms, which is equals to `Math.E` in the *Java Standard Library*. |

Example:

```   sql
select s1, s2, const(s1, 'value'='1024', 'type'='INT64'), pi(s2), e(s1, s2) from root.sg1.d1; 
```

Result:

```
select s1, s2, const(s1, 'value'='1024', 'type'='INT64'), pi(s2), e(s1, s2) from root.sg1.d1; 
+-----------------------------+--------------+--------------+-----------------------------------------------------+------------------+---------------------------------+
|                         Time|root.sg1.d1.s1|root.sg1.d1.s2|const(root.sg1.d1.s1, "value"="1024", "type"="INT64")|pi(root.sg1.d1.s2)|e(root.sg1.d1.s1, root.sg1.d1.s2)|
+-----------------------------+--------------+--------------+-----------------------------------------------------+------------------+---------------------------------+
|1970-01-01T08:00:00.000+08:00|           0.0|           0.0|                                                 1024| 3.141592653589793|                2.718281828459045|
|1970-01-01T08:00:00.001+08:00|           1.0|          null|                                                 1024|              null|                2.718281828459045|
|1970-01-01T08:00:00.002+08:00|           2.0|          null|                                                 1024|              null|                2.718281828459045|
|1970-01-01T08:00:00.003+08:00|          null|           3.0|                                                 null| 3.141592653589793|                2.718281828459045|
|1970-01-01T08:00:00.004+08:00|          null|           4.0|                                                 null| 3.141592653589793|                2.718281828459045|
+-----------------------------+--------------+--------------+-----------------------------------------------------+------------------+---------------------------------+
Total line number = 5
It costs 0.005s
```

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## Selector Functions

Currently, IoTDB supports the following selector functions:

| Function Name | Allowed Input Series Data Types       | Required Attributes                                          | Output Series Data Type       | Description                                                  |
| ------------- | ------------------------------------- | ------------------------------------------------------------ | ----------------------------- | ------------------------------------------------------------ |
| TOP_K         | INT32 / INT64 / FLOAT / DOUBLE / TEXT | `k`: the maximum number of selected data points, must be greater than 0 and less than or equal to 1000 | Same type as the input series | Returns `k` data points with the largest values in a time series. |
| BOTTOM_K      | INT32 / INT64 / FLOAT / DOUBLE / TEXT | `k`: the maximum number of selected data points, must be greater than 0 and less than or equal to 1000 | Same type as the input series | Returns `k` data points with the smallest values in a time series. |

Example：

```   sql
select s1, top_k(s1, 'k'='2'), bottom_k(s1, 'k'='2') from root.sg1.d2 where time > 2020-12-10T20:36:15.530+08:00;
```

Result：

``` 
+-----------------------------+--------------------+------------------------------+---------------------------------+
|                         Time|      root.sg1.d2.s1|top_k(root.sg1.d2.s1, "k"="2")|bottom_k(root.sg1.d2.s1, "k"="2")|
+-----------------------------+--------------------+------------------------------+---------------------------------+
|2020-12-10T20:36:15.531+08:00| 1531604122307244742|           1531604122307244742|                             null|
|2020-12-10T20:36:15.532+08:00|-7426070874923281101|                          null|                             null|
|2020-12-10T20:36:15.533+08:00|-7162825364312197604|          -7162825364312197604|                             null|
|2020-12-10T20:36:15.534+08:00|-8581625725655917595|                          null|             -8581625725655917595|
|2020-12-10T20:36:15.535+08:00|-7667364751255535391|                          null|             -7667364751255535391|
+-----------------------------+--------------------+------------------------------+---------------------------------+
Total line number = 5
It costs 0.006s
```

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    or more contributor license agreements.  See the NOTICE file
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    to you under the Apache License, Version 2.0 (the
    "License"); you may not use this file except in compliance
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## Continuous Interval Functions

The continuous interval functions are used to query all continuous intervals that meet specified conditions.
They can be divided into two categories according to return value:
1. Returns the start timestamp and time span of the continuous interval that meets the conditions (a time span of 0 means that only the start time point meets the conditions)
2. Returns the start timestamp of the continuous interval that meets the condition and the number of points in the interval (a number of 1 means that only the start time point meets the conditions)

| Function Name     | Allowed Input Series Data Types      | Required Attributes                                          | Output Series Data Type | Description                                                  |
| ----------------- | ------------------------------------ | ------------------------------------------------------------ | ----------------------- | ------------------------------------------------------------ |
| ZERO_DURATION     | INT32/ INT64/ FLOAT/ DOUBLE/ BOOLEAN | `min`:Optional with default value `0L`<br>`max`:Optional with default value `Long.MAX_VALUE` | Long                    | Return intervals' start times and duration times in which the value is always 0(false), and the duration time `t` satisfy `t >= min && t <= max`. The unit of `t` is ms |
| NON_ZERO_DURATION | INT32/ INT64/ FLOAT/ DOUBLE/ BOOLEAN | `min`:Optional with default value `0L`<br>`max`:Optional with default value `Long.MAX_VALUE` | Long                    | Return intervals' start times and duration times in which the value is always not 0, and the duration time `t` satisfy `t >= min && t <= max`. The unit of `t` is ms |
| ZERO_COUNT        | INT32/ INT64/ FLOAT/ DOUBLE/ BOOLEAN | `min`:Optional with default value `1L`<br>`max`:Optional with default value `Long.MAX_VALUE` | Long                    | Return intervals' start times and the number of data points in the interval in which the value is always 0(false). Data points number `n` satisfy `n >= min && n <= max` |
| NON_ZERO_COUNT    | INT32/ INT64/ FLOAT/ DOUBLE/ BOOLEAN | `min`:Optional with default value `1L`<br>`max`:Optional with default value `Long.MAX_VALUE` | Long                    | Return intervals' start times and the number of data points in the interval in which the value is always not 0(false). Data points number `n` satisfy `n >= min && n <= max` |

### Demonstrate
Example data:
```
IoTDB> select s1,s2,s3,s4,s5 from root.sg.d2;
+-----------------------------+-------------+-------------+-------------+-------------+-------------+
|                         Time|root.sg.d2.s1|root.sg.d2.s2|root.sg.d2.s3|root.sg.d2.s4|root.sg.d2.s5|
+-----------------------------+-------------+-------------+-------------+-------------+-------------+
|1970-01-01T08:00:00.000+08:00|            0|            0|          0.0|          0.0|        false|
|1970-01-01T08:00:00.001+08:00|            1|            1|          1.0|          1.0|         true|
|1970-01-01T08:00:00.002+08:00|            1|            1|          1.0|          1.0|         true|
|1970-01-01T08:00:00.003+08:00|            0|            0|          0.0|          0.0|        false|
|1970-01-01T08:00:00.004+08:00|            1|            1|          1.0|          1.0|         true|
|1970-01-01T08:00:00.005+08:00|            0|            0|          0.0|          0.0|        false|
|1970-01-01T08:00:00.006+08:00|            0|            0|          0.0|          0.0|        false|
|1970-01-01T08:00:00.007+08:00|            1|            1|          1.0|          1.0|         true|
+-----------------------------+-------------+-------------+-------------+-------------+-------------+
```

Sql:
```sql
select s1, zero_count(s1), non_zero_count(s2), zero_duration(s3), non_zero_duration(s4) from root.sg.d2;
```

Result:
```
+-----------------------------+-------------+-------------------------+-----------------------------+----------------------------+--------------------------------+
|                         Time|root.sg.d2.s1|zero_count(root.sg.d2.s1)|non_zero_count(root.sg.d2.s2)|zero_duration(root.sg.d2.s3)|non_zero_duration(root.sg.d2.s4)|
+-----------------------------+-------------+-------------------------+-----------------------------+----------------------------+--------------------------------+
|1970-01-01T08:00:00.000+08:00|            0|                        1|                         null|                           0|                            null|
|1970-01-01T08:00:00.001+08:00|            1|                     null|                            2|                        null|                               1|
|1970-01-01T08:00:00.002+08:00|            1|                     null|                         null|                        null|                            null|
|1970-01-01T08:00:00.003+08:00|            0|                        1|                         null|                           0|                            null|
|1970-01-01T08:00:00.004+08:00|            1|                     null|                            1|                        null|                               0|
|1970-01-01T08:00:00.005+08:00|            0|                        2|                         null|                           1|                            null|
|1970-01-01T08:00:00.006+08:00|            0|                     null|                         null|                        null|                            null|
|1970-01-01T08:00:00.007+08:00|            1|                     null|                            1|                        null|                               0|
+-----------------------------+-------------+-------------------------+-----------------------------+----------------------------+--------------------------------+
```

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    or more contributor license agreements.  See the NOTICE file
    distributed with this work for additional information
    regarding copyright ownership.  The ASF licenses this file
    to you under the Apache License, Version 2.0 (the
    "License"); you may not use this file except in compliance
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## Variation Trend Calculation Functions

Currently, IoTDB supports the following variation trend calculation functions:

| Function Name           | Allowed Input Series Data Types                 | Required Attributes                                          | Output Series Data Type       | Description                                                  |
| ----------------------- | ----------------------------------------------- | ------------------------------------------------------------ | ----------------------------- | ------------------------------------------------------------ |
| TIME_DIFFERENCE         | INT32 / INT64 / FLOAT / DOUBLE / BOOLEAN / TEXT | /                                                            | INT64                         | Calculates the difference between the time stamp of a data point and the time stamp of the previous data point. There is no corresponding output for the first data point. |
| DIFFERENCE              | INT32 / INT64 / FLOAT / DOUBLE                  | /                                                            | Same type as the input series | Calculates the difference between the value of a data point and the value of the previous data point. There is no corresponding output for the first data point. |
| NON_NEGATIVE_DIFFERENCE | INT32 / INT64 / FLOAT / DOUBLE                  | /                                                            | Same type as the input series | Calculates the absolute value of the difference between the value of a data point and the value of the previous data point. There is no corresponding output for the first data point. |
| DERIVATIVE              | INT32 / INT64 / FLOAT / DOUBLE                  | /                                                            | DOUBLE                        | Calculates the rate of change of a data point compared to the previous data point, the result is equals to DIFFERENCE / TIME_DIFFERENCE. There is no corresponding output for the first data point. |
| NON_NEGATIVE_DERIVATIVE | INT32 / INT64 / FLOAT / DOUBLE                  | /                                                            | DOUBLE                        | Calculates the absolute value of the rate of change of a data point compared to the previous data point, the result is equals to NON_NEGATIVE_DIFFERENCE / TIME_DIFFERENCE. There is no corresponding output for the first data point. |
| DIFF                    | INT32 / INT64 / FLOAT / DOUBLE                  | `ignoreNull`：optional，default is true. If is true, the previous data point is ignored when it is null and continues to find the first non-null value forwardly. If the value is false, previous data point is not ignored when it is null, the result is also null because null is used for subtraction | DOUBLE                        | Calculates the difference between the value of a data point and the value of the previous data point. There is no corresponding output for the first data point, so output is null |

Example:

```   sql
select s1, time_difference(s1), difference(s1), non_negative_difference(s1), derivative(s1), non_negative_derivative(s1) from root.sg1.d1 limit 5 offset 1000; 
```

Result:

``` 
+-----------------------------+-------------------+-------------------------------+--------------------------+---------------------------------------+--------------------------+---------------------------------------+
|                         Time|     root.sg1.d1.s1|time_difference(root.sg1.d1.s1)|difference(root.sg1.d1.s1)|non_negative_difference(root.sg1.d1.s1)|derivative(root.sg1.d1.s1)|non_negative_derivative(root.sg1.d1.s1)|
+-----------------------------+-------------------+-------------------------------+--------------------------+---------------------------------------+--------------------------+---------------------------------------+
|2020-12-10T17:11:49.037+08:00|7360723084922759782|                              1|      -8431715764844238876|                    8431715764844238876|    -8.4317157648442388E18|                  8.4317157648442388E18|
|2020-12-10T17:11:49.038+08:00|4377791063319964531|                              1|      -2982932021602795251|                    2982932021602795251|     -2.982932021602795E18|                   2.982932021602795E18|
|2020-12-10T17:11:49.039+08:00|7972485567734642915|                              1|       3594694504414678384|                    3594694504414678384|     3.5946945044146785E18|                  3.5946945044146785E18|
|2020-12-10T17:11:49.040+08:00|2508858212791964081|                              1|      -5463627354942678834|                    5463627354942678834|     -5.463627354942679E18|                   5.463627354942679E18|
|2020-12-10T17:11:49.041+08:00|2817297431185141819|                              1|        308439218393177738|                     308439218393177738|     3.0843921839317773E17|                  3.0843921839317773E17|
+-----------------------------+-------------------+-------------------------------+--------------------------+---------------------------------------+--------------------------+---------------------------------------+
Total line number = 5
It costs 0.014s
```

### Example

#### RawData

``` 
+-----------------------------+------------+------------+
|                         Time|root.test.s1|root.test.s2|
+-----------------------------+------------+------------+
|1970-01-01T08:00:00.001+08:00|           1|         1.0|
|1970-01-01T08:00:00.002+08:00|           2|        null|
|1970-01-01T08:00:00.003+08:00|        null|         3.0|
|1970-01-01T08:00:00.004+08:00|           4|        null|
|1970-01-01T08:00:00.005+08:00|           5|         5.0|
|1970-01-01T08:00:00.006+08:00|        null|         6.0|
+-----------------------------+------------+------------+
```

#### Not use `ignoreNull` attribute (Ignore Null)

SQL:
```sql
SELECT DIFF(s1), DIFF(s2) from root.test;
```

Result:
```
+-----------------------------+------------------+------------------+
|                         Time|DIFF(root.test.s1)|DIFF(root.test.s2)|
+-----------------------------+------------------+------------------+
|1970-01-01T08:00:00.001+08:00|              null|              null|
|1970-01-01T08:00:00.002+08:00|               1.0|              null|
|1970-01-01T08:00:00.003+08:00|              null|               2.0|
|1970-01-01T08:00:00.004+08:00|               2.0|              null|
|1970-01-01T08:00:00.005+08:00|               1.0|               2.0|
|1970-01-01T08:00:00.006+08:00|              null|               1.0|
+-----------------------------+------------------+------------------+
```

#### Use `ignoreNull` attribute

SQL:
```sql
SELECT DIFF(s1, 'ignoreNull'='false'), DIFF(s2, 'ignoreNull'='false') from root.test;
```

Result:
```
+-----------------------------+----------------------------------------+----------------------------------------+
|                         Time|DIFF(root.test.s1, "ignoreNull"="false")|DIFF(root.test.s2, "ignoreNull"="false")|
+-----------------------------+----------------------------------------+----------------------------------------+
|1970-01-01T08:00:00.001+08:00|                                    null|                                    null|
|1970-01-01T08:00:00.002+08:00|                                     1.0|                                    null|
|1970-01-01T08:00:00.003+08:00|                                    null|                                    null|
|1970-01-01T08:00:00.004+08:00|                                    null|                                    null|
|1970-01-01T08:00:00.005+08:00|                                     1.0|                                    null|
|1970-01-01T08:00:00.006+08:00|                                    null|                                     1.0|
+-----------------------------+----------------------------------------+----------------------------------------+
```

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    or more contributor license agreements.  See the NOTICE file
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    regarding copyright ownership.  The ASF licenses this file
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## Sample Functions

### Equal Size Bucket Sample Function

This function samples the input sequence in equal size buckets, that is, according to the downsampling ratio and downsampling method given by the user, the input sequence is equally divided into several buckets according to a fixed number of points. Sampling by the given sampling method within each bucket.
- `proportion`: sample ratio, the value range is `(0, 1]`.
#### Equal Size Bucket Random Sample
Random sampling is performed on the equally divided buckets.

| Function Name                   | Allowed Input Series Data Types | Required Attributes                                          | Output Series Data Type        | Description                                                  |
|----------|--------------------------------|---------------------------------------|------------|--------------------------------------------------|
| EQUAL_SIZE_BUCKET_RANDOM_SAMPLE   | INT32 / INT64 / FLOAT / DOUBLE | `proportion` The value range is `(0, 1]`, the default is `0.1` | INT32 / INT64 / FLOAT / DOUBLE | Returns a random sample of equal buckets that matches the sampling ratio |

##### Example

Example data: `root.ln.wf01.wt01.temperature` has a total of `100` ordered data from `0.0-99.0`.

```sql
IoTDB> select temperature from root.ln.wf01.wt01;
+-----------------------------+-----------------------------+
|                         Time|root.ln.wf01.wt01.temperature|
+-----------------------------+-----------------------------+
|1970-01-01T08:00:00.000+08:00|                          0.0|
|1970-01-01T08:00:00.001+08:00|                          1.0|
|1970-01-01T08:00:00.002+08:00|                          2.0|
|1970-01-01T08:00:00.003+08:00|                          3.0|
|1970-01-01T08:00:00.004+08:00|                          4.0|
|1970-01-01T08:00:00.005+08:00|                          5.0|
|1970-01-01T08:00:00.006+08:00|                          6.0|
|1970-01-01T08:00:00.007+08:00|                          7.0|
|1970-01-01T08:00:00.008+08:00|                          8.0|
|1970-01-01T08:00:00.009+08:00|                          9.0|
|1970-01-01T08:00:00.010+08:00|                         10.0|
|1970-01-01T08:00:00.011+08:00|                         11.0|
|1970-01-01T08:00:00.012+08:00|                         12.0|
|.............................|.............................|            
|1970-01-01T08:00:00.089+08:00|                         89.0|
|1970-01-01T08:00:00.090+08:00|                         90.0|
|1970-01-01T08:00:00.091+08:00|                         91.0|
|1970-01-01T08:00:00.092+08:00|                         92.0|
|1970-01-01T08:00:00.093+08:00|                         93.0|
|1970-01-01T08:00:00.094+08:00|                         94.0|
|1970-01-01T08:00:00.095+08:00|                         95.0|
|1970-01-01T08:00:00.096+08:00|                         96.0|
|1970-01-01T08:00:00.097+08:00|                         97.0|
|1970-01-01T08:00:00.098+08:00|                         98.0|
|1970-01-01T08:00:00.099+08:00|                         99.0|
+-----------------------------+-----------------------------+
```
Sql:
```sql
select equal_size_bucket_random_sample(temperature,'proportion'='0.1') as random_sample from root.ln.wf01.wt01;
```
Result:
```sql
+-----------------------------+-------------+
|                         Time|random_sample|
+-----------------------------+-------------+
|1970-01-01T08:00:00.007+08:00|          7.0|
|1970-01-01T08:00:00.014+08:00|         14.0|
|1970-01-01T08:00:00.020+08:00|         20.0|
|1970-01-01T08:00:00.035+08:00|         35.0|
|1970-01-01T08:00:00.047+08:00|         47.0|
|1970-01-01T08:00:00.059+08:00|         59.0|
|1970-01-01T08:00:00.063+08:00|         63.0|
|1970-01-01T08:00:00.079+08:00|         79.0|
|1970-01-01T08:00:00.086+08:00|         86.0|
|1970-01-01T08:00:00.096+08:00|         96.0|
+-----------------------------+-------------+
Total line number = 10
It costs 0.024s
```

#### Equal Size Bucket Aggregation Sample

The input sequence is sampled by the aggregation sampling method, and the user needs to provide an additional aggregation function parameter, namely
- `type`: Aggregate type, which can be `avg` or `max` or `min` or `sum` or `extreme` or `variance`. By default, `avg` is used. `extreme` represents the value with the largest absolute value in the equal bucket. `variance` represents the variance in the sampling equal buckets.

The timestamp of the sampling output of each bucket is the timestamp of the first point of the bucket.

| Function Name                | Allowed Input Series Data Types | Required Attributes                                          | Output Series Data Type        | Description                                                  |
|----------|--------------------------------|---------------------------------------|------------|--------------------------------------------------|
| EQUAL_SIZE_BUCKET_AGG_SAMPLE | INT32 / INT64 / FLOAT / DOUBLE | `proportion` The value range is `(0, 1]`, the default is `0.1`<br>`type`: The value types are `avg`, `max`, `min`, `sum`, `extreme`, `variance`, the default is `avg` | INT32 / INT64 / FLOAT / DOUBLE | Returns equal bucket aggregation samples that match the sampling ratio |

##### Example

Example data: `root.ln.wf01.wt01.temperature` has a total of `100` ordered data from `0.0-99.0`, and the test data is randomly sampled in equal buckets.

Sql:
```sql
select equal_size_bucket_agg_sample(temperature, 'type'='avg','proportion'='0.1') as agg_avg, equal_size_bucket_agg_sample(temperature, 'type'='max','proportion'='0.1') as agg_max, equal_size_bucket_agg_sample(temperature,'type'='min','proportion'='0.1') as agg_min, equal_size_bucket_agg_sample(temperature, 'type'='sum','proportion'='0.1') as agg_sum, equal_size_bucket_agg_sample(temperature, 'type'='extreme','proportion'='0.1') as agg_extreme, equal_size_bucket_agg_sample(temperature, 'type'='variance','proportion'='0.1') as agg_variance from root.ln.wf01.wt01;
```
Result:
```sql
+-----------------------------+-----------------+-------+-------+-------+-----------+------------+
|                         Time|          agg_avg|agg_max|agg_min|agg_sum|agg_extreme|agg_variance|
+-----------------------------+-----------------+-------+-------+-------+-----------+------------+
|1970-01-01T08:00:00.000+08:00|              4.5|    9.0|    0.0|   45.0|        9.0|        8.25|
|1970-01-01T08:00:00.010+08:00|             14.5|   19.0|   10.0|  145.0|       19.0|        8.25|
|1970-01-01T08:00:00.020+08:00|             24.5|   29.0|   20.0|  245.0|       29.0|        8.25|
|1970-01-01T08:00:00.030+08:00|             34.5|   39.0|   30.0|  345.0|       39.0|        8.25|
|1970-01-01T08:00:00.040+08:00|             44.5|   49.0|   40.0|  445.0|       49.0|        8.25|
|1970-01-01T08:00:00.050+08:00|             54.5|   59.0|   50.0|  545.0|       59.0|        8.25|
|1970-01-01T08:00:00.060+08:00|             64.5|   69.0|   60.0|  645.0|       69.0|        8.25|
|1970-01-01T08:00:00.070+08:00|74.50000000000001|   79.0|   70.0|  745.0|       79.0|        8.25|
|1970-01-01T08:00:00.080+08:00|             84.5|   89.0|   80.0|  845.0|       89.0|        8.25|
|1970-01-01T08:00:00.090+08:00|             94.5|   99.0|   90.0|  945.0|       99.0|        8.25|
+-----------------------------+-----------------+-------+-------+-------+-----------+------------+
Total line number = 10
It costs 0.044s
```

#### Equal Size Bucket M4 Sample

The input sequence is sampled using the M4 sampling method. That is to sample the head, tail, min and max values for each bucket.

| Function Name               | Allowed Input Series Data Types | Required Attributes                                          | Output Series Data Type        | Description                                                  |
|----------|--------------------------------|---------------------------------------|------------|--------------------------------------------------|
| EQUAL_SIZE_BUCKET_M4_SAMPLE | INT32 / INT64 / FLOAT / DOUBLE | `proportion` The value range is `(0, 1]`, the default is `0.1` | INT32 / INT64 / FLOAT / DOUBLE | Returns equal bucket M4 samples that match the sampling ratio |

##### Example

Example data: `root.ln.wf01.wt01.temperature` has a total of `100` ordered data from `0.0-99.0`, and the test data is randomly sampled in equal buckets.

Sql:
```sql
select equal_size_bucket_m4_sample(temperature, 'proportion'='0.1') as M4_sample from root.ln.wf01.wt01;
```
Result:
```sql
+-----------------------------+---------+
|                         Time|M4_sample|
+-----------------------------+---------+
|1970-01-01T08:00:00.000+08:00|      0.0|
|1970-01-01T08:00:00.001+08:00|      1.0|
|1970-01-01T08:00:00.038+08:00|     38.0|
|1970-01-01T08:00:00.039+08:00|     39.0|
|1970-01-01T08:00:00.040+08:00|     40.0|
|1970-01-01T08:00:00.041+08:00|     41.0|
|1970-01-01T08:00:00.078+08:00|     78.0|
|1970-01-01T08:00:00.079+08:00|     79.0|
|1970-01-01T08:00:00.080+08:00|     80.0|
|1970-01-01T08:00:00.081+08:00|     81.0|
|1970-01-01T08:00:00.098+08:00|     98.0|
|1970-01-01T08:00:00.099+08:00|     99.0|
+-----------------------------+---------+
Total line number = 12
It costs 0.065s
```

#### Equal Size Bucket Outlier Sample

This function samples the input sequence with equal number of bucket outliers, that is, according to the downsampling ratio given by the user and the number of samples in the bucket, the input sequence is divided into several buckets according to a fixed number of points. Sampling by the given outlier sampling method within each bucket.

| Function Name                    | Allowed Input Series Data Types | Required Attributes                                          | Output Series Data Type        | Description                                                  |
|----------|--------------------------------|---------------------------------------|------------|--------------------------------------------------|
| EQUAL_SIZE_BUCKET_OUTLIER_SAMPLE | INT32 / INT64 / FLOAT / DOUBLE | The value range of `proportion` is `(0, 1]`, the default is `0.1`<br> The value of `type` is `avg` or `stendis` or `cos` or `prenextdis`, the default is `avg` <br>The value of `number` should be greater than 0, the default is `3`| INT32 / INT64 / FLOAT / DOUBLE | Returns outlier samples in equal buckets that match the sampling ratio and the number of samples in the bucket |

Parameter Description
- `proportion`: sampling ratio
- `number`: the number of samples in each bucket, default `3`
- `type`: outlier sampling method, the value is
  - `avg`: Take the average of the data points in the bucket, and find the `top number` farthest from the average according to the sampling ratio
  - `stendis`: Take the vertical distance between each data point in the bucket and the first and last data points of the bucket to form a straight line, and according to the sampling ratio, find the `top number` with the largest distance
  - `cos`: Set a data point in the bucket as b, the data point on the left of b as a, and the data point on the right of b as c, then take the cosine value of the angle between the ab and bc vectors. The larger the angle, the more likely it is an outlier. Find the `top number` with the smallest cos value
  - `prenextdis`: Let a data point in the bucket be b, the data point to the left of b is a, and the data point to the right of b is c, then take the sum of the lengths of ab and bc as the yardstick, the larger the sum, the more likely it is to be an outlier, and find the `top number` with the largest sum value

##### Example

Example data: `root.ln.wf01.wt01.temperature` has a total of `100` ordered data from `0.0-99.0`. Among them, in order to add outliers, we make the number modulo 5 equal to 0 increment by 100.

```sql
IoTDB> select temperature from root.ln.wf01.wt01;
+-----------------------------+-----------------------------+
|                         Time|root.ln.wf01.wt01.temperature|
+-----------------------------+-----------------------------+
|1970-01-01T08:00:00.000+08:00|                          0.0|
|1970-01-01T08:00:00.001+08:00|                          1.0|
|1970-01-01T08:00:00.002+08:00|                          2.0|
|1970-01-01T08:00:00.003+08:00|                          3.0|
|1970-01-01T08:00:00.004+08:00|                          4.0|
|1970-01-01T08:00:00.005+08:00|                        105.0|
|1970-01-01T08:00:00.006+08:00|                          6.0|
|1970-01-01T08:00:00.007+08:00|                          7.0|
|1970-01-01T08:00:00.008+08:00|                          8.0|
|1970-01-01T08:00:00.009+08:00|                          9.0|
|1970-01-01T08:00:00.010+08:00|                         10.0|
|1970-01-01T08:00:00.011+08:00|                         11.0|
|1970-01-01T08:00:00.012+08:00|                         12.0|
|1970-01-01T08:00:00.013+08:00|                         13.0|
|1970-01-01T08:00:00.014+08:00|                         14.0|
|1970-01-01T08:00:00.015+08:00|                        115.0|
|1970-01-01T08:00:00.016+08:00|                         16.0|
|.............................|.............................|
|1970-01-01T08:00:00.092+08:00|                         92.0|
|1970-01-01T08:00:00.093+08:00|                         93.0|
|1970-01-01T08:00:00.094+08:00|                         94.0|
|1970-01-01T08:00:00.095+08:00|                        195.0|
|1970-01-01T08:00:00.096+08:00|                         96.0|
|1970-01-01T08:00:00.097+08:00|                         97.0|
|1970-01-01T08:00:00.098+08:00|                         98.0|
|1970-01-01T08:00:00.099+08:00|                         99.0|
+-----------------------------+-----------------------------+
```
Sql:
```sql
select equal_size_bucket_outlier_sample(temperature, 'proportion'='0.1', 'type'='avg', 'number'='2') as outlier_avg_sample, equal_size_bucket_outlier_sample(temperature, 'proportion'='0.1', 'type'='stendis', 'number'='2') as outlier_stendis_sample, equal_size_bucket_outlier_sample(temperature, 'proportion'='0.1', 'type'='cos', 'number'='2') as outlier_cos_sample, equal_size_bucket_outlier_sample(temperature, 'proportion'='0.1', 'type'='prenextdis', 'number'='2') as outlier_prenextdis_sample from root.ln.wf01.wt01;
```
Result:
```sql
+-----------------------------+------------------+----------------------+------------------+-------------------------+
|                         Time|outlier_avg_sample|outlier_stendis_sample|outlier_cos_sample|outlier_prenextdis_sample|
+-----------------------------+------------------+----------------------+------------------+-------------------------+
|1970-01-01T08:00:00.005+08:00|             105.0|                 105.0|             105.0|                    105.0|
|1970-01-01T08:00:00.015+08:00|             115.0|                 115.0|             115.0|                    115.0|
|1970-01-01T08:00:00.025+08:00|             125.0|                 125.0|             125.0|                    125.0|
|1970-01-01T08:00:00.035+08:00|             135.0|                 135.0|             135.0|                    135.0|
|1970-01-01T08:00:00.045+08:00|             145.0|                 145.0|             145.0|                    145.0|
|1970-01-01T08:00:00.055+08:00|             155.0|                 155.0|             155.0|                    155.0|
|1970-01-01T08:00:00.065+08:00|             165.0|                 165.0|             165.0|                    165.0|
|1970-01-01T08:00:00.075+08:00|             175.0|                 175.0|             175.0|                    175.0|
|1970-01-01T08:00:00.085+08:00|             185.0|                 185.0|             185.0|                    185.0|
|1970-01-01T08:00:00.095+08:00|             195.0|                 195.0|             195.0|                    195.0|
+-----------------------------+------------------+----------------------+------------------+-------------------------+
Total line number = 10
It costs 0.041s
```

### M4 Function

M4 is used to sample the `first, last, bottom, top` points for each sliding window:

-   the first point is the point with the **m**inimal time;
-   the last point is the point with the **m**aximal time;
-   the bottom point is the point with the **m**inimal value (if there are multiple such points, M4 returns one of them);
-   the top point is the point with the **m**aximal value (if there are multiple such points, M4 returns one of them).

<img src="/img/github/198178733-a0919d17-0663-4672-9c4f-1efad6f463c2.png" alt="image" style="zoom:50%;" />

| Function Name | Allowed Input Series Data Types | Required Attributes                                          | Output Series Data Type        | Description                                                  |
| ------------- | ------------------------------- | ------------------------------------------------------------ | ------------------------------ | ------------------------------------------------------------ |
| M4            | INT32 / INT64 / FLOAT / DOUBLE  | Different attributes used by the size window and the time window. The size window uses attributes `windowSize` and `slidingStep`. The time window uses attributes `timeInterval`, `slidingStep`, `displayWindowBegin`, and `displayWindowEnd`. More details see below. | INT32 / INT64 / FLOAT / DOUBLE | Returns the `first, last, bottom, top` points in each sliding window. M4 sorts and deduplicates the aggregated points within the window before outputting them. |

#### Attributes

**(1) Attributes for the size window:**

+ `windowSize`: The number of points in a window. Int data type. **Required**.
+ `slidingStep`: Slide a window by the number of points. Int data type. Optional. If not set, default to the same as `windowSize`.

<img src="/img/github/198181449-00d563c8-7bce-4ecd-a031-ec120ca42c3f.png" alt="image" style="zoom: 50%;" />

**(2) Attributes for the time window:**

+ `timeInterval`: The time interval length of a window. Unit: millisecond (ms). Long data type. **Required**.
+ `slidingStep`: Slide a window by the time length. Long data type. Optional. If not set, default to the same as `timeInterval`.
+ `displayWindowBegin`: The starting position of the window (included). Long data type. Optional. If not set, default to Long.MIN_VALUE, meaning using the time of the first data point of the input time series as the starting position of the window.
+ `displayWindowEnd`: End time limit (excluded, essentially playing the same role as `WHERE time < displayWindowEnd`). Long data type. Optional. If not set, default to Long.MAX_VALUE, meaning there is no additional end time limit other than the end of the input time series itself.

<img src="/img/github/198183015-93b56644-3330-4acf-ae9e-d718a02b5f4c.png" alt="groupBy window" style="zoom: 67%;" />

#### Examples

Input series:

```sql
+-----------------------------+------------------+
|                         Time|root.vehicle.d1.s1|
+-----------------------------+------------------+
|1970-01-01T08:00:00.001+08:00|               5.0|
|1970-01-01T08:00:00.002+08:00|              15.0|
|1970-01-01T08:00:00.005+08:00|              10.0|
|1970-01-01T08:00:00.008+08:00|               8.0|
|1970-01-01T08:00:00.010+08:00|              30.0|
|1970-01-01T08:00:00.020+08:00|              20.0|
|1970-01-01T08:00:00.025+08:00|               8.0|
|1970-01-01T08:00:00.027+08:00|              20.0|
|1970-01-01T08:00:00.030+08:00|              40.0|
|1970-01-01T08:00:00.033+08:00|               9.0|
|1970-01-01T08:00:00.035+08:00|              10.0|
|1970-01-01T08:00:00.040+08:00|              20.0|
|1970-01-01T08:00:00.045+08:00|              30.0|
|1970-01-01T08:00:00.052+08:00|               8.0|
|1970-01-01T08:00:00.054+08:00|              18.0|
+-----------------------------+------------------+
```

SQL for query1:

```sql
select M4(s1,'timeInterval'='25','displayWindowBegin'='0','displayWindowEnd'='100') from root.vehicle.d1
```

Output1:

```sql
+-----------------------------+-----------------------------------------------------------------------------------------------+
|                         Time|M4(root.vehicle.d1.s1, "timeInterval"="25", "displayWindowBegin"="0", "displayWindowEnd"="100")|
+-----------------------------+-----------------------------------------------------------------------------------------------+
|1970-01-01T08:00:00.001+08:00|                                                                                            5.0|
|1970-01-01T08:00:00.010+08:00|                                                                                           30.0|
|1970-01-01T08:00:00.020+08:00|                                                                                           20.0|
|1970-01-01T08:00:00.025+08:00|                                                                                            8.0|
|1970-01-01T08:00:00.030+08:00|                                                                                           40.0|
|1970-01-01T08:00:00.045+08:00|                                                                                           30.0|
|1970-01-01T08:00:00.052+08:00|                                                                                            8.0|
|1970-01-01T08:00:00.054+08:00|                                                                                           18.0|
+-----------------------------+-----------------------------------------------------------------------------------------------+
Total line number = 8
```

SQL for query2:

```sql
select M4(s1,'windowSize'='10') from root.vehicle.d1
```

Output2:

```sql
+-----------------------------+-----------------------------------------+
|                         Time|M4(root.vehicle.d1.s1, "windowSize"="10")|
+-----------------------------+-----------------------------------------+
|1970-01-01T08:00:00.001+08:00|                                      5.0|
|1970-01-01T08:00:00.030+08:00|                                     40.0|
|1970-01-01T08:00:00.033+08:00|                                      9.0|
|1970-01-01T08:00:00.035+08:00|                                     10.0|
|1970-01-01T08:00:00.045+08:00|                                     30.0|
|1970-01-01T08:00:00.052+08:00|                                      8.0|
|1970-01-01T08:00:00.054+08:00|                                     18.0|
+-----------------------------+-----------------------------------------+
Total line number = 7
```

#### Suggested Use Cases

**(1) Use Case: Extreme-point-preserving downsampling**

As M4 aggregation selects the `first, last, bottom, top` points for each window, M4 usually preserves extreme points and thus patterns better than other downsampling methods such as Piecewise Aggregate Approximation (PAA). Therefore, if you want to downsample the time series while preserving extreme points, you may give M4 a try.

**(2) Use case: Error-free two-color line chart visualization of large-scale time series through M4 downsampling**

Referring to paper ["M4: A Visualization-Oriented Time Series Data Aggregation"](http://www.vldb.org/pvldb/vol7/p797-jugel.pdf), M4 is a downsampling method to facilitate large-scale time series visualization without deforming the shape in terms of a two-color line chart.

Given a chart of `w*h` pixels, suppose that the visualization time range of the time series is `[tqs,tqe)` and (tqe-tqs) is divisible by w, the points that fall within the  `i`-th time span `Ii=[tqs+(tqe-tqs)/w*(i-1),tqs+(tqe-tqs)/w*i)` will be drawn on the `i`-th pixel column, i=1,2,...,w. Therefore, from a visualization-driven perspective, use the sql: `"select M4(s1,'timeInterval'='(tqe-tqs)/w','displayWindowBegin'='tqs','displayWindowEnd'='tqe') from root.vehicle.d1"` to sample the `first, last, bottom, top` points for each time span. The resulting downsampled time series has no more than `4*w` points, a big reduction compared to the original large-scale time series. Meanwhile, the two-color line chart drawn from the reduced data is identical that to that drawn from the original data (pixel-level consistency).

To eliminate the hassle of hardcoding parameters, we recommend the following usage of Grafana's [template variable](https://grafana.com/docs/grafana/latest/dashboards/variables/add-template-variables/#global-variables) `$__interval_ms` when Grafana is used for visualization:

```
select M4(s1,'timeInterval'='$__interval_ms') from root.sg1.d1
```

where `timeInterval` is set as `(tqe-tqs)/w` automatically. Note that the time precision here is assumed to be milliseconds.

#### Comparison with Other Functions

| SQL                                                          | Whether support M4 aggregation                               | Sliding window type                               | Example                                                      | Docs                                                         |
| ------------------------------------------------------------ | ------------------------------------------------------------ | ------------------------------------------------- | ------------------------------------------------------------ | ------------------------------------------------------------ |
| 1. native built-in aggregate functions with Group By clause  | No. Lack `BOTTOM_TIME` and `TOP_TIME`, which are respectively the time of the points that have the mininum and maximum value. | Time Window                                       | `select count(status), max_value(temperature) from root.ln.wf01.wt01 group by ([2017-11-01 00:00:00, 2017-11-07 23:00:00), 3h, 1d)` | https://iotdb.apache.org/UserGuide/Master/Query-Data/Aggregate-Query.html#built-in-aggregate-functions <br />https://iotdb.apache.org/UserGuide/Master/Query-Data/Aggregate-Query.html#downsampling-aggregate-query |
| 2. EQUAL_SIZE_BUCKET_M4_SAMPLE (built-in UDF)                | Yes*                                                         | Size Window. `windowSize = 4*(int)(1/proportion)` | `select equal_size_bucket_m4_sample(temperature, 'proportion'='0.1') as M4_sample from root.ln.wf01.wt01` | https://iotdb.apache.org/UserGuide/Master/Query-Data/Select-Expression.html#time-series-generating-functions |
| **3. M4 (built-in UDF)**                                     | Yes*                                                         | Size Window, Time Window                          | (1) Size Window: `select M4(s1,'windowSize'='10') from root.vehicle.d1` <br />(2) Time Window: `select M4(s1,'timeInterval'='25','displayWindowBegin'='0','displayWindowEnd'='100') from root.vehicle.d1` | refer to this doc                                            |
| 4. extend native built-in aggregate functions with Group By clause to support M4 aggregation | not implemented                                              | not implemented                                   | not implemented                                              | not implemented                                              |

Further compare `EQUAL_SIZE_BUCKET_M4_SAMPLE` and `M4`:

**(1) Different M4 aggregation definition:**

For each window, `EQUAL_SIZE_BUCKET_M4_SAMPLE` extracts the top and bottom points from points **EXCLUDING** the first and last points.

In contrast, `M4` extracts the top and bottom points from points **INCLUDING** the first and last points, which is more consistent with the semantics of `max_value` and `min_value` stored in metadata.

It is worth noting that both functions sort and deduplicate the aggregated points in a window before outputting them to the collectors.

**(2) Different sliding windows:** 

`EQUAL_SIZE_BUCKET_M4_SAMPLE` uses SlidingSizeWindowAccessStrategy and **indirectly** controls sliding window size by sampling proportion. The conversion formula is `windowSize = 4*(int)(1/proportion)`. 

`M4` supports two types of sliding window: SlidingSizeWindowAccessStrategy and SlidingTimeWindowAccessStrategy. `M4` **directly** controls the window point size or time length using corresponding parameters.


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-->

## Time Series Processing

### CHANGE_POINTS

#### Usage

This function is used to remove consecutive identical values from an input sequence.
For example, input:`1，1，2，2，3` output:`1，2，3`.

**Name：** CHANGE_POINTS

**Input Series：** Support only one input series.

**Parameters：** No parameters.

#### Example

Raw data：

```
+-----------------------------+---------------------------+---------------------------+---------------------------+---------------------------+---------------------------+---------------------------+
|                         Time|root.testChangePoints.d1.s1|root.testChangePoints.d1.s2|root.testChangePoints.d1.s3|root.testChangePoints.d1.s4|root.testChangePoints.d1.s5|root.testChangePoints.d1.s6|
+-----------------------------+---------------------------+---------------------------+---------------------------+---------------------------+---------------------------+---------------------------+
|1970-01-01T08:00:00.001+08:00|                       true|                          1|                          1|                        1.0|                        1.0|                     1test1|
|1970-01-01T08:00:00.002+08:00|                       true|                          2|                          2|                        2.0|                        1.0|                     2test2|
|1970-01-01T08:00:00.003+08:00|                      false|                          1|                          2|                        1.0|                        1.0|                     2test2|
|1970-01-01T08:00:00.004+08:00|                       true|                          1|                          3|                        1.0|                        1.0|                     1test1|
|1970-01-01T08:00:00.005+08:00|                       true|                          1|                          3|                        1.0|                        1.0|                     1test1|
+-----------------------------+---------------------------+---------------------------+---------------------------+---------------------------+---------------------------+---------------------------+
```

SQL for query：

```sql
select change_points(s1), change_points(s2), change_points(s3), change_points(s4), change_points(s5), change_points(s6) from root.testChangePoints.d1
```

Output series：

```
+-----------------------------+------------------------------------------+------------------------------------------+------------------------------------------+------------------------------------------+------------------------------------------+------------------------------------------+
|                         Time|change_points(root.testChangePoints.d1.s1)|change_points(root.testChangePoints.d1.s2)|change_points(root.testChangePoints.d1.s3)|change_points(root.testChangePoints.d1.s4)|change_points(root.testChangePoints.d1.s5)|change_points(root.testChangePoints.d1.s6)|
+-----------------------------+------------------------------------------+------------------------------------------+------------------------------------------+------------------------------------------+------------------------------------------+------------------------------------------+
|1970-01-01T08:00:00.001+08:00|                                      true|                                         1|                                         1|                                       1.0|                                       1.0|                                    1test1|
|1970-01-01T08:00:00.002+08:00|                                      null|                                         2|                                         2|                                       2.0|                                      null|                                    2test2|
|1970-01-01T08:00:00.003+08:00|                                     false|                                         1|                                      null|                                       1.0|                                      null|                                      null|
|1970-01-01T08:00:00.004+08:00|                                      true|                                      null|                                         3|                                      null|                                      null|                                    1test1|
+-----------------------------+------------------------------------------+------------------------------------------+------------------------------------------+------------------------------------------+------------------------------------------+------------------------------------------+
```


## Conditional Expressions

### CASE

The CASE expression is a kind of conditional expression that can be used to return different values based on specific conditions, similar to the if-else statements in other languages.

The CASE expression consists of the following parts:

- CASE keyword: Indicates the start of the CASE expression.
- WHEN-THEN clauses: There may be multiple clauses used to define conditions and give results. This clause is divided into two parts, WHEN and THEN. The WHEN part defines the condition, and the THEN part defines the result expression. If the WHEN condition is true, the corresponding THEN result is returned.
- ELSE clause: If none of the WHEN conditions is true, the result in the ELSE clause will be returned. The ELSE clause can be omitted.
- END keyword: Indicates the end of the CASE expression.

The CASE expression is a scalar operation that can be used in combination with any other scalar operation or aggregate function.

In the following text, all THEN parts and ELSE clauses will be collectively referred to as result clauses.

#### Syntax

The CASE expression supports two formats.

- Format 1:
    ```sql
    CASE 
        WHEN condition1 THEN expression1
        [WHEN condition2 THEN expression2] ...
        [ELSE expression_end]
    END
    ```
  The `condition`s will be evaluated one by one.

  The first `condition` that is true will return the corresponding expression.

- Format 2:
    ```sql
    CASE caseValue
        WHEN whenValue1 THEN expression1
        [WHEN whenValue2 THEN expression2] ...
        [ELSE expression_end]
    END
    ```
  The `caseValue` will be evaluated first, and then the `whenValue`s will be evaluated one by one. The first `whenValue` that is equal to the `caseValue` will return the corresponding `expression`.

  Format 2 will be transformed into an equivalent Format 1 by iotdb.

  For example, the above SQL statement will be transformed into:

    ```sql
    CASE 
        WHEN caseValue=whenValue1 THEN expression1
        [WHEN caseValue=whenValue1 THEN expression1] ...
        [ELSE expression_end]
    END
    ```

If none of the conditions are true, or if none of the `whenValue`s match the `caseValue`, the `expression_end` will be returned.

If there is no ELSE clause, `null` will be returned.

#### Notes

- In format 1, all WHEN clauses must return a BOOLEAN type.
- In format 2, all WHEN clauses must be able to be compared to the CASE clause.
- All result clauses in a CASE expression must satisfy certain conditions for their return value types:
  - BOOLEAN types cannot coexist with other types and will cause an error if present.
  - TEXT types cannot coexist with other types and will cause an error if present.
  - The other four numeric types can coexist, and the final result will be of DOUBLE type, with possible precision loss during conversion.
  - If necessary, you can use the CAST function to convert the result to a type that can coexist with others.
- The CASE expression does not implement lazy evaluation, meaning that all clauses will be evaluated.
- The CASE expression does not support mixing with UDFs.
- Aggregate functions cannot be used within a CASE expression, but the result of a CASE expression can be used as input for an aggregate function.
- When using the CLI, because the CASE expression string can be lengthy, it is recommended to provide an alias for the expression using AS.

#### Using Examples

##### Example 1

The CASE expression can be used to analyze data in a visual way. For example:
- The preparation of a certain chemical product requires that the temperature and pressure be within specific ranges.
- During the preparation process, sensors will detect the temperature and pressure, forming two time-series T (temperature) and P (pressure) in IoTDB.
In this application scenario, the CASE expression can indicate which time parameters are appropriate, which are not, and why they are not.

data:
```sql
IoTDB> select * from root.test1
+-----------------------------+------------+------------+
|                         Time|root.test1.P|root.test1.T|
+-----------------------------+------------+------------+
|2023-03-29T11:25:54.724+08:00|   1000000.0|      1025.0|
|2023-03-29T11:26:13.445+08:00|   1000094.0|      1040.0|
|2023-03-29T11:27:36.988+08:00|   1000095.0|      1041.0|
|2023-03-29T11:27:56.446+08:00|   1000095.0|      1059.0|
|2023-03-29T11:28:20.838+08:00|   1200000.0|      1040.0|
+-----------------------------+------------+------------+
```

SQL statements:
```sql
select T, P, case
when 1000<T and T<1050 and 1000000<P and P<1100000 then "good!"
when T<=1000 or T>=1050 then "bad temperature"
when P<=1000000 or P>=1100000 then "bad pressure"
end as `result`
from root.test1
```


output:
```
+-----------------------------+------------+------------+---------------+
|                         Time|root.test1.T|root.test1.P|         result|
+-----------------------------+------------+------------+---------------+
|2023-03-29T11:25:54.724+08:00|      1025.0|   1000000.0|   bad pressure|
|2023-03-29T11:26:13.445+08:00|      1040.0|   1000094.0|          good!|
|2023-03-29T11:27:36.988+08:00|      1041.0|   1000095.0|          good!|
|2023-03-29T11:27:56.446+08:00|      1059.0|   1000095.0|bad temperature|
|2023-03-29T11:28:20.838+08:00|      1040.0|   1200000.0|   bad pressure|
+-----------------------------+------------+------------+---------------+
```


##### Example 2

The CASE expression can achieve flexible result transformation, such as converting strings with a certain pattern to other strings.

data:
```sql
IoTDB> select * from root.test2
+-----------------------------+--------------+
|                         Time|root.test2.str|
+-----------------------------+--------------+
|2023-03-27T18:23:33.427+08:00|         abccd|
|2023-03-27T18:23:39.389+08:00|         abcdd|
|2023-03-27T18:23:43.463+08:00|       abcdefg|
+-----------------------------+--------------+
```

SQL statements:
```sql
select str, case
when str like "%cc%" then "has cc"
when str like "%dd%" then "has dd"
else "no cc and dd" end as `result`
from root.test2
```

output:
```
+-----------------------------+--------------+------------+
|                         Time|root.test2.str|      result|
+-----------------------------+--------------+------------+
|2023-03-27T18:23:33.427+08:00|         abccd|      has cc|
|2023-03-27T18:23:39.389+08:00|         abcdd|      has dd|
|2023-03-27T18:23:43.463+08:00|       abcdefg|no cc and dd|
+-----------------------------+--------------+------------+
```

##### Example 3: work with aggregation functions

###### Valid: aggregation function ← CASE expression

The CASE expression can be used as a parameter for aggregate functions. For example, used in conjunction with the COUNT function, it can implement statistics based on multiple conditions simultaneously.

data:
```sql
IoTDB> select * from root.test3
+-----------------------------+------------+
|                         Time|root.test3.x|
+-----------------------------+------------+
|2023-03-27T18:11:11.300+08:00|         0.0|
|2023-03-27T18:11:14.658+08:00|         1.0|
|2023-03-27T18:11:15.981+08:00|         2.0|
|2023-03-27T18:11:17.668+08:00|         3.0|
|2023-03-27T18:11:19.112+08:00|         4.0|
|2023-03-27T18:11:20.822+08:00|         5.0|
|2023-03-27T18:11:22.462+08:00|         6.0|
|2023-03-27T18:11:24.174+08:00|         7.0|
|2023-03-27T18:11:25.858+08:00|         8.0|
|2023-03-27T18:11:27.979+08:00|         9.0|
+-----------------------------+------------+
```

SQL statements:

```sql
select
count(case when x<=1 then 1 end) as `(-∞,1]`,
count(case when 1<x and x<=3 then 1 end) as `(1,3]`,
count(case when 3<x and x<=7 then 1 end) as `(3,7]`,
count(case when 7<x then 1 end) as `(7,+∞)`
from root.test3
```

output:
```
+------+-----+-----+------+
|(-∞,1]|(1,3]|(3,7]|(7,+∞)|
+------+-----+-----+------+
|     2|    2|    4|     2|
+------+-----+-----+------+
```

###### Invalid: CASE expression ← aggregation function 

Using aggregation function in CASE expression is not supported

SQL statements:
```sql
select case when x<=1 then avg(x) else sum(x) end from root.test3
```

output:
```
Msg: 701: Raw data and aggregation result hybrid calculation is not supported.
```

##### Example 4: kind 2

Here is a simple example that uses the format 2 syntax. 
If all conditions are equality tests, it is recommended to use format 2 to simplify SQL statements.

data:
```sql
IoTDB> select * from root.test4
+-----------------------------+------------+
|                         Time|root.test4.x|
+-----------------------------+------------+
|1970-01-01T08:00:00.001+08:00|         1.0|
|1970-01-01T08:00:00.002+08:00|         2.0|
|1970-01-01T08:00:00.003+08:00|         3.0|
|1970-01-01T08:00:00.004+08:00|         4.0|
+-----------------------------+------------+
```

SQL statements:
```sql
select x, case x when 1 then "one" when 2 then "two" else "other" end from root.test4
```

output:
```
+-----------------------------+------------+-----------------------------------------------------------------------------------+
|                         Time|root.test4.x|CASE WHEN root.test4.x = 1 THEN "one" WHEN root.test4.x = 2 THEN "two" ELSE "other"|
+-----------------------------+------------+-----------------------------------------------------------------------------------+
|1970-01-01T08:00:00.001+08:00|         1.0|                                                                                one|
|1970-01-01T08:00:00.002+08:00|         2.0|                                                                                two|
|1970-01-01T08:00:00.003+08:00|         3.0|                                                                              other|
|1970-01-01T08:00:00.004+08:00|         4.0|                                                                              other|
+-----------------------------+------------+-----------------------------------------------------------------------------------+
```

##### Example 5: type of return clauses

The result clause of a CASE expression needs to satisfy certain type restrictions.

In this example, we continue to use the data from Example 4.

###### Invalid: BOOLEAN cannot coexist with other types

SQL statements:
```sql
select x, case x when 1 then true when 2 then 2 end from root.test4
```

output:
```
Msg: 701: CASE expression: BOOLEAN and other types cannot exist at same time
```

###### Valid: Only BOOLEAN type exists

SQL statements:
```sql
select x, case x when 1 then true when 2 then false end as `result` from root.test4
```

output:
```
+-----------------------------+------------+------+
|                         Time|root.test4.x|result|
+-----------------------------+------------+------+
|1970-01-01T08:00:00.001+08:00|         1.0|  true|
|1970-01-01T08:00:00.002+08:00|         2.0| false|
|1970-01-01T08:00:00.003+08:00|         3.0|  null|
|1970-01-01T08:00:00.004+08:00|         4.0|  null|
+-----------------------------+------------+------+
```

###### Invalid:TEXT cannot coexist with other types

SQL statements:
```sql
select x, case x when 1 then 1 when 2 then "str" end from root.test4
```

output:
```
Msg: 701: CASE expression: TEXT and other types cannot exist at same time
```

###### Valid: Only TEXT type exists

See in Example 1.

###### Valid: Numerical types coexist

SQL statements:
```sql
select x, case x
when 1 then 1
when 2 then 222222222222222
when 3 then 3.3
when 4 then 4.4444444444444
end as `result`
from root.test4
```

output:
```
+-----------------------------+------------+-------------------+
|                         Time|root.test4.x|             result|
+-----------------------------+------------+-------------------+
|1970-01-01T08:00:00.001+08:00|         1.0|                1.0|
|1970-01-01T08:00:00.002+08:00|         2.0|2.22222222222222E14|
|1970-01-01T08:00:00.003+08:00|         3.0|  3.299999952316284|
|1970-01-01T08:00:00.004+08:00|         4.0|   4.44444465637207|
+-----------------------------+------------+-------------------+
```