src/zh/UserGuide/latest-Table/SQL-Manual/Identifier.md

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# 标识符

在 IoTDB 中，标识符用于标识 database、table、column、function 或其他对象名称。

## 1. 命名规则

- __开头字符__：标识符必须以字母或下划线开头。
- __后续字符__：可以包含字母、数字和下划线。
- __特殊字符__：如果标识符包含除字母、数字和下划线之外的其他字符，必须用双引号（`"`）括起来。
- __转义字符__：在双引号定界的标识符中，使用两个连续的双引号（`""`）来表示一个双引号字符。

### 1.1 示例

以下是一些有效的标识符示例：

```sql
test
"table$partitions"
"identifierWith"
"double"
"quotes"
```

无效的标识符示例，使用时必须用双引号引用：

```sql
table-name // 包含短横线
123SchemaName // 以数字开头
colum$name@field  // 包含特殊字符且未用双引号括起
```

## 2. 大小写敏感性

标识符不区分大小写，且系统存储标识符时不保留原始大小写，查询结果会根据用户在`SELECT`子句中指定的大小写显示列名。

> 双引号括起来的标识符也不区分大小写。

### 2.1 示例

当创建了一个名为 `Device_id` 的列，在查看表时看到为 `device_id`，但返回的结果列与用户查询时指定的格式保持相同为`Device_ID`：

```sql
IoTDB> create table table1(Device_id STRING TAG, Model STRING ATTRIBUTE, TemPerature FLOAT FIELD, Humidity DOUBLE FIELD)

IoTDB> desc table1;
+-----------+---------+-----------+
| ColumnName| DataType|   Category|
+-----------+---------+-----------+
|       time|TIMESTAMP|       TIME|
|  device_id|   STRING|        TAG|
|      model|   STRING|  ATTRIBUTE|
|temperature|    FLOAT|      FIELD|
|   humidity|   DOUBLE|      FIELD|
+-----------+---------+-----------+

IoTDB> select TiMe, Device_ID, MoDEL, TEMPerature, HUMIdity from table1;
+-----------------------------+---------+------+-----------+--------+
|                         TiMe|Device_ID| MoDEL|TEMPerature|HUMIdity|
+-----------------------------+---------+------+-----------+--------+
|1970-01-01T08:00:00.001+08:00|       d1|modelY|       27.2|    67.0|
+-----------------------------+---------+------+-----------+--------+
```