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> `EXISTS` 运算符文档

# EXISTS

`EXISTS` 运算符用于检查子查询结果中是否有记录。如果结果为空，则返回 `0`；否则返回 `1`。

`EXISTS` 也可用于 [WHERE](/zh/reference/statements/select/where) 子句。

<Tip>
  ***

  子查询不支持引用主查询中的表和列。
</Tip>

**语法**

```sql theme={null}
EXISTS(subquery)
```

**示例**

检查子查询中值是否存在的查询：

```sql title="Query" theme={null}
SELECT EXISTS(SELECT * FROM numbers(10) WHERE number > 8), EXISTS(SELECT * FROM numbers(10) WHERE number > 11)
```

```text title="Response" theme={null}
┌─in(1, _subquery1)─┬─in(1, _subquery2)─┐
│                 1 │                 0 │
└───────────────────┴───────────────────┘
```

包含返回多行的子查询的查询：

```sql title="Query" theme={null}
SELECT count() FROM numbers(10) WHERE EXISTS(SELECT number FROM numbers(10) WHERE number > 8);
```

```text title="Response" theme={null}
┌─count()─┐
│      10 │
└─────────┘
```

带有返回空结果的子查询的查询：

```sql title="Query" theme={null}
SELECT count() FROM numbers(10) WHERE EXISTS(SELECT number FROM numbers(10) WHERE number > 11);
```

```text title="Response" theme={null}
┌─count()─┐
│       0 │
└─────────┘
```