Source: https://dev-docs.yueshu.com.cn/v5.3.2/database-gql-reference/fe/predicates/exists/
Documentation version: v5.3.2

# EXISTS 谓词

`EXISTS`谓词检查某个图模式是否存在或某个查询的结果是否非空。

## 语法

语法图源码

 exists_predicate  

````
exists_predicate ::=
    EXISTS ( '{' graph_pattern '}' | '(' graph_pattern ')' | '{' match_statement+ '}' | '{' procedure_body '}' )

````

## 参数

### `graph_pattern`

待检查的图模式。更多信息，请参见[图模式](https://dev-docs.yueshu.com.cn/v5.3.2/database-gql-reference/patterns/)。

### `match_statement`

指定待检查的查询的 MATCH 语句。更多信息，请参见 [MATCH](https://dev-docs.yueshu.com.cn/v5.3.2/database-gql-reference/dql/match/)。

### `procedure_body`

待检查的过程的处理逻辑，可以由[原始值变量定义](https://dev-docs.yueshu.com.cn/v5.3.2/database-gql-reference/variable-definition/value-variable/)和以下类型的语句组成：

- [数据定义语句](https://dev-docs.yueshu.com.cn/v5.3.2/database-gql-reference/dql/)
- [控制流语句](https://dev-docs.yueshu.com.cn/v5.3.2/database-gql-reference/control-flow/)
- [变量操作语句](https://dev-docs.yueshu.com.cn/v5.3.2/database-gql-reference/set/)
- [日志语句](https://dev-docs.yueshu.com.cn/v5.3.2/database-gql-reference/log/)

## 示例

以下示例基于[示例数据集](https://dev-docs.yueshu.com.cn/v5.3.2/database-gql-reference/overview/example-dataset/)。

### 检查点模式是否存在

````
'''
USE movie 
RETURN EXISTS { (v WHERE v.id= 123 ) } AS a
'''

````

返回：

````
+------+
| a    |
+------+
| true |
+------+

````

### 检查边模式是否存在

````
'''
USE movie 
RETURN EXISTS ( ()-[e]->() WHERE e.id= 13423 ) AS b
'''

````

返回：

````
Error: [NS230]: property `id` not found in EDGE...

````

### 检查查询的结果是否非空

````
'''
USE movie 
RETURN EXISTS { MATCH (a@`User`)-[e:Watch]->(b) WHERE a.id = 123 RETURN e } AS c 
'''

````

返回：

````
+------+
| c    |
+------+
| true |
+------+

````

### 检查过程的结果是否非空

````
'''
USE movie
MATCH (a@Actor) WHERE a.id IN [1, 2, 3, 4, 5]
LET d = EXISTS {
  VALUE born_before_1950::BOOL
  IF a.birthDate < DATE "1950-01-01" THEN 
    {    
      SET born_before_1950 = true
      LOG_INFO(a.name, " was born before 1950: ", born_before_1950)
      FILTER born_before_1950 RETURN born_before_1950
    }
  ELSE 
    { 
      SET born_before_1950 = false
      LOG_INFO(a.name, " was born before 1950: ", born_before_1950)
      FILTER born_before_1950 RETURN born_before_1950
    }
}   
RETURN a.name AS actor_name, d AS born_before_1950
'''

````

上述实例检查 ID 在 1 到 5 之间的每个演员是否在 1950 年前出生，并记录结果。如果该过程返回非空结果，则`EXISTS`谓词返回`true`。

返回：

````
+---------------+------------------+
| actor_name    | born_before_1950 |
+---------------+------------------+
| Mark Hamill   | false            |
| Carrie Fisher | false            |
| Peter Cushing | true             |
| Harrison Ford | true             |
+---------------+------------------+
Log:
2026-01-20T07:44:00.840764 INFO  - Carrie Fisher was born before 1950: false
2026-01-20T07:44:00.840794 INFO  - Mark Hamill was born before 1950: false
2026-01-20T07:44:00.840804 INFO  - Peter Cushing was born before 1950: true
2026-01-20T07:44:00.841090 INFO  - Harrison Ford was born before 1950: true

````