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# Syntax Rule

## Literal Values

This section describes how to write literal values in IoTDB. These include strings, numbers, timestamp values, boolean values, and NULL.

### String Literals

in IoTDB, **A string is a sequence of bytes or characters, enclosed within either single quote (`'`) or double quote (`"`) characters.** Examples：

```js
'a string'
"another string"
```

#### Usage Scenarios

Usages of string literals:

- Values of  `TEXT` type data in `INSERT` or `SELECT` statements 

    ```sql
    # insert
    insert into root.ln.wf02.wt02(timestamp,hardware) values(1, 'v1')
    insert into root.ln.wf02.wt02(timestamp,hardware) values(2, '\\')
    
    +-----------------------------+--------------------------+
    |                         Time|root.ln.wf02.wt02.hardware|
    +-----------------------------+--------------------------+
    |1970-01-01T08:00:00.001+08:00|                        v1|
    +-----------------------------+--------------------------+
    |1970-01-01T08:00:00.002+08:00|                        \\|
    +-----------------------------+--------------------------+
    
    # select
    select code from root.sg1.d1 where code in ('string1', 'string2');
    ```

- Used in`LOAD` / `REMOVE` / `SETTLE` instructions to represent file path.

    ```sql
    # load
    LOAD 'examplePath'
    
    # remove
    REMOVE 'examplePath'
    
    # SETTLE
    SETTLE 'examplePath'
    ```

- Password fields in user management statements

    ```sql
    # write_pwd is the password
    CREATE USER ln_write_user 'write_pwd'
    ```

- Full Java class names in UDF and trigger management statements 

    ```sql
    # Trigger example. Full java class names after 'AS' should be string literals.
    CREATE TRIGGER `alert-listener-sg1d1s1`
    AFTER INSERT
    ON root.sg1.d1.s1
    AS 'org.apache.iotdb.db.engine.trigger.example.AlertListener'
    WITH (
      'lo' = '0', 
      'hi' = '100.0'
    )
    
    # UDF example. Full java class names after 'AS' should be string literals.
    CREATE FUNCTION example AS 'org.apache.iotdb.udf.UDTFExample'
    ```

- `AS` function provided by IoTDB can assign an alias to time series selected in query. Alias can be constant(including string) or identifier.

    ```sql
    select s1 as 'temperature', s2 as 'speed' from root.ln.wf01.wt01;
    
    # Header of dataset
    +-----------------------------+-----------|-----+
    |                         Time|temperature|speed|
    +-----------------------------+-----------|-----+
    ```

- The key/value of an attribute can be String Literal and identifier, more details can be found at **key-value pair** part. 


#### How to use quotation marks in String Literals

There are several ways to include quote characters within a string:

 - `'` inside a string quoted with `"` needs no special treatment and need not be doubled or escaped. In the same way, `"` inside a string quoted with `'` needs no special treatment.
 - A `'` inside a string quoted with `'` may be written as `''`.
- A `"` inside a string quoted with `"` may be written as `""`.

The following examples demonstrate how quoting and escaping work:

```js
'string'  // string
'"string"'  // "string"
'""string""'  // ""string""
'''string'  // 'string

"string" // string
"'string'"  // 'string'
"''string''"  // ''string''
"""string"  // "string
```

### Numeric Literals

Number literals include integer (exact-value) literals and floating-point (approximate-value) literals.

Integers are represented as a sequence of digits. Numbers may be preceded by `-` or `+` to indicate a negative or positive value, respectively. Examples: `1`, `-1`.

Numbers with fractional part or represented in scientific notation with a mantissa and exponent are approximate-value numbers. Examples: `.1`, `3.14`, `-2.23`, `+1.70`, `1.2E3`, `1.2E-3`, `-1.2E3`, `-1.2E-3`.

The `INT32` and `INT64` data types are integer types and calculations are exact.

The `FLOAT` and `DOUBLE` data types are floating-point types and calculations are approximate.

An integer may be used in floating-point context; it is interpreted as the equivalent floating-point number.

### Timestamp Literals

The timestamp is the time point at which data is produced. It includes absolute timestamps and relative timestamps in IoTDB. For information about timestamp support in IoTDB, see [Data Type Doc](../Basic-Concept/Data-Type.md).

Specially, `NOW()` represents a constant timestamp that indicates the system time at which the statement began to execute.

### Boolean Literals

The constants `TRUE` and `FALSE` evaluate to 1 and 0, respectively. The constant names can be written in any lettercase.

### NULL Values

The `NULL` value means “no data.” `NULL` can be written in any lettercase.

## Identifier

### Usage scenarios

Certain objects within IoTDB, including `TRIGGER`, `FUNCTION`(UDF), `CONTINUOUS QUERY`, `SCHEMA TEMPLATE`, `USER`, `ROLE`,`Pipe`,`PipeSink`,`alias` and other object names are known as identifiers.

### Constraints

Below are basic constraints of identifiers, specific identifiers may have other constraints, for example, `user` should consists of more than 4 characters. 

- Permitted characters in unquoted identifiers:
    - [0-9 a-z A-Z _ ] (letters, digits and underscore)
    - ['\u2E80'..'\u9FFF'] (UNICODE Chinese characters)
- Identifiers may begin with a digit, unquoted identifiers can not be a real number.
- Identifiers are case sensitive.
- Key words can be used as an identifier.

**You need to quote the identifier with back quote(`) in the following cases:**

- Identifier contains special characters.
- Identifier that is a real number.

### How to use quotations marks in quoted identifiers

`'` and `"` can be used directly in quoted identifiers.

` may be written as `` in quoted  identifiers. See the example below:

```sql
# create template t1't"t
create schema template `t1't"t` 
(temperature FLOAT encoding=RLE, status BOOLEAN encoding=PLAIN compression=SNAPPY)

# create template t1`t
create schema template `t1``t` 
(temperature FLOAT encoding=RLE, status BOOLEAN encoding=PLAIN compression=SNAPPY)
```

### Examples

Examples of case in which quoted identifier is used ：

- Trigger name should be quoted in cases described above ：

    ```sql
    # create trigger named alert.`listener-sg1d1s1
    CREATE TRIGGER `alert.``listener-sg1d1s1`
    AFTER INSERT
    ON root.sg1.d1.s1
    AS 'org.apache.iotdb.db.storageengine.trigger.example.AlertListener'
    WITH (
      'lo' = '0', 
      'hi' = '100.0'
    )
    ```

- UDF name should be quoted in cases described above ：

    ```sql
    # create a funciton named 111, 111 is a real number.
    CREATE FUNCTION `111` AS 'org.apache.iotdb.udf.UDTFExample'
    ```

- Template name should be quoted in cases described above ：

    ```sql
    # create a template named 111, 111 is a real number.
    create schema template `111` 
    (temperature FLOAT encoding=RLE, status BOOLEAN encoding=PLAIN compression=SNAPPY)
    ```

- User and Role name should be quoted in cases described above, blank space is not allow in User and Role name whether quoted or not ：

    ```sql
    # create user special`user.
    CREATE USER `special``user.` 'write_pwd'
    
    # create role 111
    CREATE ROLE `111`
    ```

- Continuous query name should be quoted in cases described above ：

    ```sql
    # create continuous query test.cq
    CREATE CONTINUOUS QUERY `test.cq` 
    BEGIN 
      SELECT max_value(temperature) 
      INTO temperature_max 
      FROM root.ln.*.* 
      GROUP BY time(10s) 
    END
    ```

- Pipe、PipeSink should be quoted in cases described above ：

    ```sql
    # create PipeSink test.*1
    CREATE PIPESINK `test.*1` AS IoTDB ('ip' = '输入你的IP')
    
    # create Pipe test.*2
    CREATE PIPE `test.*2` TO `test.*1` FROM 
    (select ** from root WHERE time>=yyyy-mm-dd HH:MM:SS) WITH 'SyncDelOp' = 'true'
    ```

- `AS` function provided by IoTDB can assign an alias to time series selected in query. Alias can be constant(including string) or identifier.

    ```sql
    select s1 as temperature, s2 as speed from root.ln.wf01.wt01;
    
    # Header of result dataset
    +-----------------------------+-----------|-----+
    |                         Time|temperature|speed|
    +-----------------------------+-----------|-----+
    ```

- The key/value of an attribute can be String Literal and identifier, more details can be found at **key-value pair** part. 
  
- Nodes except database in the path are allowed to contain the "*" symbol, when using this symbol it is required to enclose the node in backquotes, e.g., root.db.`*`, but this usage is only recommended when the path cannot avoid containing the "*" symbol. 

## KeyWords Words

Keywords are words that have significance in SQL. Keywords can be used as an identifier. Certain keywords, such as TIME/TIMESTAMP and ROOT, are reserved and cannot use as identifiers.

[Keywords](../Reference/Keywords.md) shows the keywords in IoTDB.

## Detailed Definitions of Lexical and Grammar

Please read the lexical and grammar description files in our code repository:

Lexical file: `antlr/src/main/antlr4/org/apache/iotdb/db/qp/sql/IoTDBSqlLexer.g4`

Grammer file: `antlr/src/main/antlr4/org/apache/iotdb/db/qp/sql/IoTDBSqlParser.g4`