src/zh/UserGuide/latest-Table/SQL-Manual/Tableless-Query_apache.md

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# 无表查询

## 1. 功能概述

无表查询支持在不依赖实际数据库表的情况下，直接执行不包含 `FROM` 子句的 SQL 查询语句，获取计算结果、常量值或系统信息，主要用于表达式计算、系统函数调用、常量查询等场景。

> 注意：自 V2.0.11 版本起支持该功能。

## 2. 语法定义

```sql
SELECT expression [ [ AS ] column_alias ]
    [, expression [ [ AS ] column_alias ] ]*;
```

其中：

- `expression`：待计算的表达式，可以是常量、数学函数、字符串函数、日期时间函数、聚合函数，目前支持**无参数或只需传入常量参数**的函数。

- `column_alias`：可选的结果列别名，用于提升查询结果的可读性。未指定别名时，系统会自动生成 `_col0`、`_col1` 等列名。

- 不支持使用 `SELECT *`。

- 执行 `SELECT COUNT(*);` 时，结果始终为 `1`。

## 3. 使用示例

### 3.1 常量与数学计算

查询语句：

```sql
SELECT 100 + 50 AS sum_result,
  10 * 5 AS product,
  SQRT(144) AS square_root,
  ABS(-25) AS absolute_value,
  SIN(1) AS sin_value;
```

返回结果：

```text
+----------+-------+-----------+--------------+------------------+
|sum_result|product|square_root|absolute_value|         sin_value|
+----------+-------+-----------+--------------+------------------+
|       150|     50|       12.0|            25|0.8414709848078965|
+----------+-------+-----------+--------------+------------------+
```

### 3.2 字符串函数

查询语句：

```sql
SELECT
  CONCAT('Hello', ' ', 'World') AS greeting,
  UPPER('iotdb') AS uppercase,
  LOWER('IOTDB') AS lowercase,
  LENGTH('database') AS str_length;
```

返回结果：

```text
+-----------+---------+---------+----------+
|   greeting|uppercase|lowercase|str_length|
+-----------+---------+---------+----------+
|Hello World|    IOTDB|    iotdb|         8|
+-----------+---------+---------+----------+
```

### 3.3 日期时间函数

查询语句：

```sql
SELECT NOW() AS now_time;
```

返回结果：

```text
+-------------+
|     now_time|
+-------------+
|1784776724555|
+-------------+
```

### 3.4 聚合函数

查询语句：

```sql
SELECT
  AVG(10),
  SUM(10),
  COUNT(10),
  COUNT(*);
```

返回结果：

```text
+-----+-----+-----+-----+
|_col0|_col1|_col2|_col3|
+-----+-----+-----+-----+
| 10.0| 10.0|    1|    1|
+-----+-----+-----+-----+
```