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# 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.engine.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.