docs/UserGuide/Syntax-Conventions/Literal-Values.md

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# 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](../Data-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.