Subqueries with EXISTS or NOT EXISTS

语法描述

EXISTS 用于检查子查询是否至少会返回一行数据。即将主查询的数据，放到子查询中做条件验证，根据验证结果（TRUE 或 FALSE）来决定主查询的数据结果是否得以保留。

如果子查询返回任何行，EXISTS 子查询条件为 TRUE，NOT EXISTS 子查询条件为 FALSE。

语法结构

1. > SELECT column_name(s)
2. FROM table_name
3. WHERE EXISTS
4. (SELECT column_name FROM table_name WHERE condition);

示例

1. create table t1 (a int);
2. create table t2 (a int, b int);
3. create table t3 (a int);
4. create table t4 (a int not null, b int not null);
5. insert into t1 values (2);
6. insert into t2 values (1,7),(2,7);
7. insert into t4 values (4,8),(3,8),(5,9);
8. insert into t3 values (6),(7),(3);

10. mysql> select * from t3 where exists (select * from t2 where t2.b=t3.a);
11. +------+
12. | a    |
13. +------+
14. |    7 |
15. +------+
16. 1 row in set (0.00 sec)
17. mysql> select * from t3 where not exists (select * from t2 where t2.b=t3.a);
18. +------+
19. | a    |
20. +------+
21. |    6 |
22. |    3 |
23. +------+
24. 2 rows in set (0.00 sec)