SQL中的子查询_百度小程序_上海毫米网络优化公司

一、子查询定义

当我们进⾏语句查询的时候，总会遇到我们需要的条件需要通过另⼀个查询语句查询出来后才能进⾏，就是说A 查询语句需要依赖B 查询语句的查询结果，B 查询就是⼦查询，A 查询语句就是主查询，⼀个SQL语句可以包含多个⼦查询。

二、子查询出现的地方

- 在where⼦句中: ⼦查询的结果可⽤作条件筛选时使⽤的值。
- 在from⼦句中: ⼦查询的结果可充当⼀张表或视图，需要使⽤表别名。
- 在having⼦句中: ⼦查询的结果可⽤作分组查询再次条件过滤时使⽤的值
- 在select⼦句中: ⼦查询的结果可充当⼀个字段。仅限⼦查询返回单⾏单列的情况。

1、子查询出现在where子句中

1、查询⼯资⼤于10号部⻔的平均⼯资的⾮10号部⻔的员⼯信息
分析：
1）10号部⻔的平均⼯资
 select avg(sal) from emp where deptno = 10;
2）查询员工信息
 select * from emp where deptno != 10 and sal > (select avg(sal) from emp where deptno = 10);
 
+-------+-------+---------+------+------------+---------+------+--------+
| empno | ename | job     | mgr  | hiredate   | sal     | comm | deptno |
+-------+-------+---------+------+------------+---------+------+--------+
|  7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL |     20 |
|  7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL |     20 |
|  7902 | FORD  | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL |     20 |
+-------+-------+---------+------+------------+---------+------+--------+
 
 
2、查询与7369同部⻔的同事信息
 
select deptno from emp where empno = 7369;
 
select * from emp where deptno = (select deptno from emp where empno = 7369)
  and empno != 7369;
 
+-------+-------+---------+------+------------+---------+------+--------+
| empno | ename | job     | mgr  | hiredate   | sal     | comm | deptno |
+-------+-------+---------+------+------------+---------+------+--------+
|  7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL |     20 |
|  7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL |     20 |
|  7876 | ADAMS | CLERK   | 7788 | 1987-05-23 | 1100.00 | NULL |     20 |
|  7902 | FORD  | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL |     20 |
+-------+-------+---------+------+------------+---------+------+--------+

2、子查询出现在from子句中

1、查询员⼯的姓名，⼯资，及其部⻔的平均⼯资
 select ename,sal from emp ;
 思考：
  每一个部门的平均工资是多少？
  select deptno,avg(sal) from emp group by deptno;
+--------+-------------+
| deptno | avg(sal)    |
+--------+-------------+
|     10 | 2916.666667 |
|     20 | 2175.000000 |
|     30 | 1566.666667 |
|    100 | NULL        |
+--------+-------------+
4 rows in set (0.09 sec)
查询员工信息：
select * from emp;
mysql> select * from emp;
+-------+--------+-----------+------+------------+---------+---------+--------+
| empno | ename  | job       | mgr  | hiredate   | sal     | comm    | deptno |
+-------+--------+-----------+------+------------+---------+---------+--------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 | 800.00  | NULL    |     20 |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 | 300.00  |     30 |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 | 500.00  |     30 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 | NULL    |     20 |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 | NULL    |     30 |
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 | NULL    |     10 |
|  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 | NULL    |     20 |
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL    |     10 |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 | 0.00    |     30 |
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 | NULL    |     20 |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 | 950.00  | NULL    |     30 |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 | NULL    |     20 |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 | NULL    |     10 |
|  8000 | LaoYan | NULL      | NULL | NULL       | NULL    | NULL    |    100 |
+-------+--------+-----------+------+------------+---------+---------+--------+
 
此时：查询出来的平均工资表可以当做一个虚拟的表，和emp表关联起来
 
mysql> select * from 
  ( select deptno,avg(sal) avgMoney from emp group by deptno ) avgTable;
+--------+-------------+
| deptno | avgMoney    |
+--------+-------------+
|     10 | 2916.666667 |
|     20 | 2175.000000 |
|     30 | 1566.666667 |
|    100 | NULL        |
+--------+-------------+
 
现在 avgTable 表  和 emp  通过 deptno 相关联
select  
   ename,sal,avgMoney from 
    emp , 
   (select deptno,avg(sal) avgMoney from emp group by deptno) avgTable   
   where emp.deptno=avgTable.deptno;
+--------+---------+-------------+
| ename  | sal     | avgMoney    |
+--------+---------+-------------+
| SMITH  | 800.00  | 2175.000000 |
| ALLEN  | 1600.00 | 1566.666667 |
| WARD   | 1250.00 | 1566.666667 |
| JONES  | 2975.00 | 2175.000000 |
| MARTIN | 1250.00 | 1566.666667 |
| BLAKE  | 2850.00 | 1566.666667 |
| CLARK  | 2450.00 | 2916.666667 |
| SCOTT  | 3000.00 | 2175.000000 |
| KING   | 5000.00 | 2916.666667 |
| TURNER | 1500.00 | 1566.666667 |
| ADAMS  | 1100.00 | 2175.000000 |
| JAMES  | 950.00  | 1566.666667 |
| FORD   | 3000.00 | 2175.000000 |
| MILLER | 1300.00 | 2916.666667 |
| LaoYan | NULL    | NULL        |
+--------+---------+-------------+
最终的答案！
 
虚拟机的表解释：--任何的查询结果都可以当做一个虚拟机的表来使用！
mysql> select ename,job,sal from emp where job is not null;
+--------+-----------+---------+
| ename  | job       | sal     |
+--------+-----------+---------+
| SMITH  | CLERK     | 800.00  |
| ALLEN  | SALESMAN  | 1600.00 |
| WARD   | SALESMAN  | 1250.00 |
| JONES  | MANAGER   | 2975.00 |
| MARTIN | SALESMAN  | 1250.00 |
| BLAKE  | MANAGER   | 2850.00 |
| CLARK  | MANAGER   | 2450.00 |
| SCOTT  | ANALYST   | 3000.00 |
| KING   | PRESIDENT | 5000.00 |
| TURNER | SALESMAN  | 1500.00 |
| ADAMS  | CLERK     | 1100.00 |
| JAMES  | CLERK     | 950.00  |
| FORD   | ANALYST   | 3000.00 |
| MILLER | CLERK     | 1300.00 |
+--------+-----------+---------+
14 rows in set (0.09 sec)
 
mysql> select * from (select ename,job,sal from emp where job is not null) eemp;
+--------+-----------+---------+
| ename  | job       | sal     |
+--------+-----------+---------+
| SMITH  | CLERK     | 800.00  |
| ALLEN  | SALESMAN  | 1600.00 |
| WARD   | SALESMAN  | 1250.00 |
| JONES  | MANAGER   | 2975.00 |
| MARTIN | SALESMAN  | 1250.00 |
| BLAKE  | MANAGER   | 2850.00 |
| CLARK  | MANAGER   | 2450.00 |
| SCOTT  | ANALYST   | 3000.00 |
| KING   | PRESIDENT | 5000.00 |
| TURNER | SALESMAN  | 1500.00 |
| ADAMS  | CLERK     | 1100.00 |
| JAMES  | CLERK     | 950.00  |
| FORD   | ANALYST   | 3000.00 |
| MILLER | CLERK     | 1300.00 |
+--------+-----------+---------+
14 rows in set (0.06 sec)

3、子查询出现在select后面

2、查询员⼯的姓名，⼯资，及其部⻔的平均⼯资
说一个情况： emp表和emp表也是可以自己关联自己的。
 select * from emp e1, emp e2 where e1.empno = e2.empno;
 
 
select ename,sal, (select avg(sal) from emp e2 where e2.deptno = e1.deptno ) 
 部门平均工资 from emp e1;
拆解：
select avg(sal) from emp e2 where e2.deptno = 10;
select avg(sal) from emp e2 where e2.deptno = 20;
 
最终的答案：
+--------+---------+--------------+
| ename  | sal     | 部门平均工资 |
+--------+---------+--------------+
| SMITH  | 800.00  | 2175.000000  |
| ALLEN  | 1600.00 | 1566.666667  |
| WARD   | 1250.00 | 1566.666667  |
| JONES  | 2975.00 | 2175.000000  |
| MARTIN | 1250.00 | 1566.666667  |
| BLAKE  | 2850.00 | 1566.666667  |
| CLARK  | 2450.00 | 2916.666667  |
| SCOTT  | 3000.00 | 2175.000000  |
| KING   | 5000.00 | 2916.666667  |
| TURNER | 1500.00 | 1566.666667  |
| ADAMS  | 1100.00 | 2175.000000  |
| JAMES  | 950.00  | 1566.666667  |
| FORD   | 3000.00 | 2175.000000  |
| MILLER | 1300.00 | 2916.666667  |
| LaoYan | NULL    | NULL         |
+--------+---------+--------------+
 
优化一下：工资 保留小数点后2位
select ename,sal, 
   round(
    ( select avg(sal) from emp e2 where e2.deptno = e1.deptno),2) 部门平均工资 
  from emp e1;
+--------+---------+--------------+
| ename  | sal     | 部门平均工资 |
+--------+---------+--------------+
| SMITH  | 800.00  | 2175.00      |
| ALLEN  | 1600.00 | 1566.67      |
| WARD   | 1250.00 | 1566.67      |
| JONES  | 2975.00 | 2175.00      |
| MARTIN | 1250.00 | 1566.67      |
| BLAKE  | 2850.00 | 1566.67      |
| CLARK  | 2450.00 | 2916.67      |
| SCOTT  | 3000.00 | 2175.00      |
| KING   | 5000.00 | 2916.67      |
| TURNER | 1500.00 | 1566.67      |
| ADAMS  | 1100.00 | 2175.00      |
| JAMES  | 950.00  | 1566.67      |
| FORD   | 3000.00 | 2175.00      |
| MILLER | 1300.00 | 2916.67      |
| LaoYan | NULL    | NULL         |
+--------+---------+--------------+
 
再来一遍，仔细品味：
mysql> select ename,sal, ( select avg(sal) from emp e2 where e2.deptno = 10 ) from emp;
+--------+---------+------------------------------------------------------+
| ename  | sal     | ( select avg(sal) from emp e2 where e2.deptno = 10 ) |
+--------+---------+------------------------------------------------------+
| SMITH  | 800.00  | 2916.666667                                          |
| ALLEN  | 1600.00 | 2916.666667                                          |
| WARD   | 1250.00 | 2916.666667                                          |
| JONES  | 2975.00 | 2916.666667                                          |
| MARTIN | 1250.00 | 2916.666667                                          |
| BLAKE  | 2850.00 | 2916.666667                                          |
| CLARK  | 2450.00 | 2916.666667                                          |
| SCOTT  | 3000.00 | 2916.666667                                          |
| KING   | 5000.00 | 2916.666667                                          |
| TURNER | 1500.00 | 2916.666667                                          |
| ADAMS  | 1100.00 | 2916.666667                                          |
| JAMES  | 950.00  | 2916.666667                                          |
| FORD   | 3000.00 | 2916.666667                                          |
| MILLER | 1300.00 | 2916.666667                                          |
| LaoYan | NULL    | 2916.666667                                          |
+--------+---------+------------------------------------------------------+
15 rows in set (0.10 sec)
 
mysql> select ename,sal, ( select avg(sal) from emp e2 where e2.deptno = emp.deptno ) from emp;
+--------+---------+--------------------------------------------------------------+
| ename  | sal     | ( select avg(sal) from emp e2 where e2.deptno = emp.deptno ) |
+--------+---------+--------------------------------------------------------------+
| SMITH  | 800.00  | 2175.000000   |
| ALLEN  | 1600.00 | 1566.666667   |
| WARD   | 1250.00 | 1566.666667   |
| JONES  | 2975.00 | 2175.000000   |
| MARTIN | 1250.00 | 1566.666667   |
| BLAKE  | 2850.00 | 1566.666667   |
| CLARK  | 2450.00 | 2916.666667   |
| SCOTT  | 3000.00 | 2175.000000   |
| KING   | 5000.00 | 2916.666667   |
| TURNER | 1500.00 | 1566.666667   |
| ADAMS  | 1100.00 | 2175.000000   |
| JAMES  | 950.00  | 1566.666667   |
| FORD   | 3000.00 | 2175.000000   |
| MILLER | 1300.00 | 2916.666667   |
| LaoYan | NULL    | NULL          |
+--------+---------+--------------------------------------------------------------+
15 rows in set (0.11 sec)
 
 
再来一题：
查询每个员⼯的信息及其部⻔的平均⼯资，⼯资之和，部⻔⼈数
select *,
  (select avg(sal) from emp e1 where e1.deptno = emp.deptno) 平均工资,
  (select sum(sal) from emp e2 where e2.deptno = emp.deptno) 工资之和,
  (select count(*) from emp e3 where e3.deptno = emp.deptno) 部门人数
from emp;
 
尝试-- 这种不行，报 Operand should contain 1 column(s)
select *,(
   select avg(sal) 平均工资,sum(sal) 工资之和,count(*) 部门人数  from emp e1 where e1.deptno = emp.deptno) from emp;
 
改变思路：在玩一种很新的东西
 
mysql> select deptno,avg(sal) 平均工资,sum(sal) 工资之和,count(*) 部门人数  from emp e1
   group by deptno ;
+--------+-------------+----------+----------+
| deptno | 平均工资    | 工资之和 | 部门人数 |
+--------+-------------+----------+----------+
|     10 | 2916.666667 | 8750.00  |        3 |
|     20 | 2175.000000 | 10875.00 |        5 |
|     30 | 1566.666667 | 9400.00  |        6 |
|    100 | NULL        | NULL     |        1 |
+--------+-------------+----------+----------+
 
硬套进去：失败了，原因是这个地方只允许出现一列数据
select * , (select * from (select deptno,avg(sal) 平均工资,sum(sal) 工资之和,count(*) 部门人数  from emp e1
   group by deptno) pgb where pgb.deptno =emp.deptno ) from emp;

4、子查询出现在having子句中

查询平均⼯资⼤于30号部⻔的平均⼯资的部⻔号，和平均⼯资
分析：30号部⻔的平均⼯资
select avg(sal) from emp where deptno = 30;
查询所有部门的平局工资：
select deptno,avg(sal) from emp group by deptno;
 
整合
select deptno,avg(sal) avgMoney from emp group by deptno 
   having avgMoney > (select avg(sal) from emp where deptno = 30);
+--------+-------------+
| deptno | avgMoney    |
+--------+-------------+
|     10 | 2916.666667 |
|     20 | 2175.000000 |
+--------+-------------+

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