mysql 数据操作 多表查询 子查询 带EXISTS关键字的子查询

带EXISTS关键字的子查询

EXISTS关字键字表示存在。

EXISTS 判断某个sql语句的有没有查到结果 有就返回真  true 否则返回假 False

如果条件成立 返回另外一条sql语句的返回结果

返回结果了

mysql> select * from employee where  EXISTS(select id from department where name='技术');
+----+------------+--------+------+--------+
| id | name       | sex    | age  | dep_id |
+----+------------+--------+------+--------+
|  1 | mike       | male   |   18 |    200 |
|  2 | alex       | female |   48 |    201 |
|  3 | jack       | male   |   38 |    201 |
|  4 | yuanhao    | female |   28 |    202 |
|  5 | liwenzhou  | male   |   18 |    200 |
|  6 | jingliyang | female |   18 |    204 |
+----+------------+--------+------+--------+
6 rows in set (0.00 sec)

子查询也查不出结果 exists 返回是false

mysql> select * from employee where  EXISTS(select id from department where name='it');
Empty set (0.00 sec)