十、子查询
10.1 什么是子查询
select语句中嵌套select语句，被嵌套的select语句称为子查询。
子查询出现的位置：
select ..(select).
from ..(select).
where ..(select).
10.2 where子句中的子查询
找出比最低工资高的员工姓名和工资？
# 错误示范：
mysql> select ename, sal
-> from emp
-> where sal > min(sal);
ERROR 1111 (HY000): Invalid use of group function
# 实现思路：
1. 查询最低工资是多少；
2. 找出 > min(sal) 的；
3. 合并
# 1. 查询最低工资是多少；
mysql> select min(sal) from emp;
+----------+
| min(sal) |
+----------+
| 800.00 |
+----------+
1 row in set (0.00 sec)
# 2. 找出 > min(sal) 的；
mysql> select ename, sal
-> from emp
-> where sal > 800;
+--------+---------+
| ename | sal |
+--------+---------+
| ALLEN | 1600.00 |
| WARD | 1250.00 |
| JONES | 2975.00 |
| MARTIN | 1250.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| SCOTT | 3000.00 |
| KING | 5000.00 |
| TURNER | 1500.00 |
| ADAMS | 1100.00 |
| JAMES | 950.00 |
| FORD | 3000.00 |
| MILLER | 1300.00 |
+--------+---------+
13 rows in set (0.00 sec)
# 3. 合并
mysql> select ename, sal
-> from emp
-> where sal > (select min(sal) from emp);
+--------+---------+
| ename | sal |
+--------+---------+
| ALLEN | 1600.00 |
| WARD | 1250.00 |
| JONES | 2975.00 |
| MARTIN | 1250.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| SCOTT | 3000.00 |
| KING | 5000.00 |
| TURNER | 1500.00 |
| ADAMS | 1100.00 |
| JAMES | 950.00 |
| FORD | 3000.00 |
| MILLER | 1300.00 |
+--------+---------+
13 rows in set (0.00 sec)
10.3 from子句中的子查询
注意：from后面的子查询，可以将子查询的查询结果当作一张临时的表（技巧）
找出每个岗位的平均工资的薪资等级？
# 第一步，找出每个岗位的平均工资
mysql> select job, avg(sal)
-> from emp
-> group by job;
+-----------+-------------+
| job | avg(sal) |
+-----------+-------------+
| ANALYST | 3000.000000 |
| CLERK | 1037.500000 |
| MANAGER | 2758.333333 |
| PRESIDENT | 5000.000000 |
| SALESMAN | 1400.000000 |
+-----------+-------------+
5 rows in set (0.00 sec)
# 第二步，把上面的查询结果当作一张真实存在的表t，下面是s表
mysql> select *
-> from salgrade;
+-------+-------+-------+
| GRADE | LOSAL | HISAL |
+-------+-------+-------+
| 1 | 700 | 1200 |
| 2 | 1201 | 1400 |
| 3 | 1401 | 2000 |
| 4 | 2001 | 3000 |
| 5 | 3001 | 9999 |
+-------+-------+-------+
5 rows in set (0.00 sec)
# 第三步，t表和s表进行表连接，条件：t表avg(sal) between s.losal and s.hisal;
mysql> select t.*, s.grade
-> from (select job, avg(sal) as avgsal from emp group by job) t
-> join salgrade s
-> on t.avgsal between s.losal and s.hisal;
+-----------+-------------+-------+
| job | avgsal | grade |
+-----------+-------------+-------+
| ANALYST | 3000.000000 | 4 |
| CLERK | 1037.500000 | 1 |
| MANAGER | 2758.333333 | 4 |
| PRESIDENT | 5000.000000 | 5 |
| SALESMAN | 1400.000000 | 2 |
+-----------+-------------+-------+
5 rows in set (0.00 sec)
10.4 select后面出现的子查询（了解即可）
找出每个员工的部门名称，要求显示员工名，部门名：
mysql> select e.ename, e.deptno, (select d.dname from dept d where e.deptno = d.deptno) as dname
-> from emp e;
+--------+--------+------------+
| ename | deptno | dname |
+--------+--------+------------+
| SMITH | 20 | RESEARCH |
| ALLEN | 30 | SALES |
| WARD | 30 | SALES |
| JONES | 20 | RESEARCH |
| MARTIN | 30 | SALES |
| BLAKE | 30 | SALES |
| CLARK | 10 | ACCOUNTING |
| SCOTT | 20 | RESEARCH |
| KING | 10 | ACCOUNTING |
| TURNER | 30 | SALES |
| ADAMS | 20 | RESEARCH |
| JAMES | 30 | SALES |
| FORD | 20 | RESEARCH |
| MILLER | 10 | ACCOUNTING |
+--------+--------+------------+
14 rows in set (0.00 sec)
# 错误示范：
mysql> select e.ename, e.deptno, (select dname from dept) as dname
-> from emp e;
ERROR 1242 (21000): Subquery returns more than 1 row
注意：对于select后面的子查询来说，这个子查询只能一次返回1条结果，多于1条就会报错。
10.5 union合并查询结果集
union的效率会高一些。对于表连接来说，每连接一次新表，则匹配的次数满足笛卡尔积，会成倍的翻。
而union可以减少匹配的次数（union把乘法变成了加法运算）。在减少匹配次数的情况下，还可以完成两个结果集的拼接。
a 连接 b 连接 c
a 10条记录
b 10条记录
c 10条记录
匹配次数：1000
a 连接 b一个结果：10 * 10 --> 100次
a 连接 c一个结果：10 * 10 --> 100次
使用union的话：100次 + 100次 = 200次。
注意：union在进行结果集合并的时候，要求两个结果集的列数相同！
查询工作岗位是MANAGER和SALESMAN的员工？
# 法1
mysql> select ename, job
-> from emp
-> where job = 'MANAGER' or job = 'SALESMAN';
+--------+----------+
| ename | job |
+--------+----------+
| ALLEN | SALESMAN |
| WARD | SALESMAN |
| JONES | MANAGER |
| MARTIN | SALESMAN |
| BLAKE | MANAGER |
| CLARK | MANAGER |
| TURNER | SALESMAN |
+--------+----------+
7 rows in set (0.00 sec)
# 法2
mysql> select ename, job
-> from emp
-> where job in('MANAGER', 'SALESMAN');
+--------+----------+
| ename | job |
+--------+----------+
| ALLEN | SALESMAN |
| WARD | SALESMAN |
| JONES | MANAGER |
| MARTIN | SALESMAN |
| BLAKE | MANAGER |
| CLARK | MANAGER |
| TURNER | SALESMAN |
+--------+----------+
7 rows in set (0.00 sec)
# 法3
mysql> select ename, job
-> from emp
-> where job = 'MANAGER'
-> union
-> select ename, job
-> from emp
-> where job = 'SALESMAN';
+--------+----------+
| ename | job |
+--------+----------+
| JONES | MANAGER |
| BLAKE | MANAGER |
| CLARK | MANAGER |
| ALLEN | SALESMAN |
| WARD | SALESMAN |
| MARTIN | SALESMAN |
| TURNER | SALESMAN |
+--------+----------+
7 rows in set (0.00 sec)
union在使用时的注意事项：
# 注意：union在进行结果集合并的时候，要求两个结果集的列数相同！
mysql> select ename,job from emp where job = 'MANAGER';
+-------+---------+
| ename | job |
+-------+---------+
| JONES | MANAGER |
| BLAKE | MANAGER |
| CLARK | MANAGER |
+-------+---------+
3 rows in set (0.00 sec)
mysql> select ename from emp where job = 'SALESMAN';
+--------+
| ename |
+--------+
| ALLEN |
| WARD |
| MARTIN |
| TURNER |
+--------+
4 rows in set (0.02 sec)
# 列数不同，报错
mysql> select ename,job from emp where job = 'MANAGER'
-> union
-> select ename from emp where job = 'SALESMAN';
ERROR 1222 (21000): The used SELECT statements have a different number of columns