子查询

1 定义

：select查询语句中嵌套了select语句，嵌套的那个select查询语句就叫子查询
1.1 从employee表中查询出工资最高员工的信息

mysql> select * from employee;
+----+------+------+--------+--------+--------+
| id | name | age  | salary | phone  | in_dpt |
+----+------+------+--------+--------+--------+
|  1 | Tom  |   26 |   2500 | 119119 | dpt4   |
|  2 | Jack |   24 |   2500 | 120120 | dpt2   |
|  3 | Rose |   22 |   2800 | 114114 | dpt3   |
|  4 | Jim  |   35 |   3000 | 100861 | dpt1   |
|  5 | Mary |   21 |   3000 | 100101 | dpt2   |
|  6 | Alex |   26 |   3000 | 123456 | dpt1   |
|  7 | Ken  |   27 |   3500 | 654321 | dpt1   |
|  8 | Rick |   24 |   3500 | 987654 | dpt3   |
|  9 | Joe  |   31 |   3600 | 110129 | dpt2   |
| 10 | Mike |   23 |   3400 | 110110 | dpt4   |
| 11 | Jobs | NULL |   3600 |  19283 | dpt2   |
| 12 | Tony | NULL |   3400 | 102938 | dpt3   |
+----+------+------+--------+--------+--------+
12 rows in set (0.00 sec)

mysql> select MAX(salary) from employee;
+-------------+
| MAX(salary) |
+-------------+
|        3600 |
+-------------+
1 row in set (0.00 sec)

mysql> select * from employee where salary =(select MAX(salary) from employee);
+----+------+------+--------+--------+--------+
| id | name | age  | salary | phone  | in_dpt |
+----+------+------+--------+--------+--------+
|  9 | Joe  |   31 |   3600 | 110129 | dpt2   |
| 11 | Jobs | NULL |   3600 |  19283 | dpt2   |
+----+------+------+--------+--------+--------+
2 rows in set (0.00 sec)

mysql> 

2 子查询的分类

2.1 子查询结果为单行单列，可以作为WHERE的条件使用运算符去判断，运算符包括 > ,<, = ,>=,<=
查询工资小于平均工资的员工信息

mysql> select avg(salary) from employee;
+-------------+
| avg(salary) |
+-------------+
|   3150.0000 |
+-------------+
1 row in set (0.00 sec)

mysql> select * from employee where salary < (select avg(salary) from employee);
+----+------+------+--------+--------+--------+
| id | name | age  | salary | phone  | in_dpt |
+----+------+------+--------+--------+--------+
|  1 | Tom  |   26 |   2500 | 119119 | dpt4   |
|  2 | Jack |   24 |   2500 | 120120 | dpt2   |
|  3 | Rose |   22 |   2800 | 114114 | dpt3   |
|  4 | Jim  |   35 |   3000 | 100861 | dpt1   |
|  5 | Mary |   21 |   3000 | 100101 | dpt2   |
|  6 | Alex |   26 |   3000 | 123456 | dpt1   |
+----+------+------+--------+--------+--------+
6 rows in set (0.00 sec)

mysql> 

2.2 子查询结果是多行单列，以作为WHERE的条件使用运算符IN去判断
查询部门人数为11和15的员工信息

mysql> select * from department;
+----------+------------+
| dpt_name | people_num |
+----------+------------+
| dpt1     |         11 |
| dpt2     |         12 |
| dpt3     |         10 |
| dpt4     |         15 |
+----------+------------+
4 rows in set (0.01 sec)

mysql> select * from employee;
+----+------+------+--------+--------+--------+
| id | name | age  | salary | phone  | in_dpt |
+----+------+------+--------+--------+--------+
|  1 | Tom  |   26 |   2500 | 119119 | dpt4   |
|  2 | Jack |   24 |   2500 | 120120 | dpt2   |
|  3 | Rose |   22 |   2800 | 114114 | dpt3   |
|  4 | Jim  |   35 |   3000 | 100861 | dpt1   |
|  5 | Mary |   21 |   3000 | 100101 | dpt2   |
|  6 | Alex |   26 |   3000 | 123456 | dpt1   |
|  7 | Ken  |   27 |   3500 | 654321 | dpt1   |
|  8 | Rick |   24 |   3500 | 987654 | dpt3   |
|  9 | Joe  |   31 |   3600 | 110129 | dpt2   |
| 10 | Mike |   23 |   3400 | 110110 | dpt4   |
| 11 | Jobs | NULL |   3600 |  19283 | dpt2   |
| 12 | Tony | NULL |   3400 | 102938 | dpt3   |
+----+------+------+--------+--------+--------+
12 rows in set (0.00 sec)

mysql> select dpt_name from department where people_num = 11 or people_num = 15;
+----------+
| dpt_name |
+----------+
| dpt1     |
| dpt4     |
+----------+
2 rows in set (0.00 sec)

mysql> select * from employee where in_dpt in(select dpt_name from department where people_num in(11,15));
+----+------+------+--------+--------+--------+
| id | name | age  | salary | phone  | in_dpt |
+----+------+------+--------+--------+--------+
|  4 | Jim  |   35 |   3000 | 100861 | dpt1   |
|  6 | Alex |   26 |   3000 | 123456 | dpt1   |
|  7 | Ken  |   27 |   3500 | 654321 | dpt1   |
|  1 | Tom  |   26 |   2500 | 119119 | dpt4   |
| 10 | Mike |   23 |   3400 | 110110 | dpt4   |
+----+------+------+--------+--------+--------+
5 rows in set (0.01 sec)

2.3 子查询结果是多行多列，可以作为一张虚拟表，和其他表连接查询。
查询工资大于平均工资的员工和其部门的信息

mysql> select * from employee where salary >(select avg(salary) from employee);
+----+------+------+--------+--------+--------+
| id | name | age  | salary | phone  | in_dpt |
+----+------+------+--------+--------+--------+
|  7 | Ken  |   27 |   3500 | 654321 | dpt1   |
|  8 | Rick |   24 |   3500 | 987654 | dpt3   |
|  9 | Joe  |   31 |   3600 | 110129 | dpt2   |
| 10 | Mike |   23 |   3400 | 110110 | dpt4   |
| 11 | Jobs | NULL |   3600 |  19283 | dpt2   |
| 12 | Tony | NULL |   3400 | 102938 | dpt3   |
+----+------+------+--------+--------+--------+
6 rows in set (0.00 sec)

mysql> select * from department t1,(select * from employee where salary >(select avg(salary) from employee)) t2 where t1.dpt_name
    -> =t2.in_dpt;
+----------+------------+----+------+------+--------+--------+--------+
| dpt_name | people_num | id | name | age  | salary | phone  | in_dpt |
+----------+------------+----+------+------+--------+--------+--------+
| dpt1     |         11 |  7 | Ken  |   27 |   3500 | 654321 | dpt1   |
| dpt3     |         10 |  8 | Rick |   24 |   3500 | 987654 | dpt3   |
| dpt2     |         12 |  9 | Joe  |   31 |   3600 | 110129 | dpt2   |
| dpt4     |         15 | 10 | Mike |   23 |   3400 | 110110 | dpt4   |
| dpt2     |         12 | 11 | Jobs | NULL |   3600 |  19283 | dpt2   |
| dpt3     |         10 | 12 | Tony | NULL |   3400 | 102938 | dpt3   |
+----------+------------+----+------+------+--------+--------+--------+
6 rows in set (0.00 sec)

mysql>