【MySQL学习笔记】（八）复合查询

在前面的笔记中做的查询基本都是对一张表进行查询，在实际开发中远远不够，本篇文章内容是复合查询相关的笔记。需要用到oracle9i的经典测试表，在笔记（六）中已经教大家如何导入了。

复合查询

• 基本查询回顾
• 多表查询
• 子连接
• 子查询
• • 单行子查询
  • 多行子查询
  • 多列子查询
  • [在 from 子句中使用子查询](#在 from 子句中使用子查询)
  • 合并查询

基本查询回顾

这是 emp 表

mysql> select* from emp;
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |
| 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |
| 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |
| 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |
| 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |
| 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |
| 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |
| 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |
| 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
14 rows in set (0.00 sec)

• 查询工资高于500或岗位为MANAGER的雇员，同时还要满足他们的姓名首字母为大写的J

mysql> select* from emp where (sal > 500 or job='MANAGER') and ename like 'J%'
+--------+-------+---------+------+---------------------+---------+------+--------+
| empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
+--------+-------+---------+------+---------------------+---------+------+--------+
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
| 007900 | JAMES | CLERK   | 7698 | 1981-12-03 00:00:00 |  950.00 | NULL |     30 |
+--------+-------+---------+------+---------------------+---------+------+--------+
2 rows in set (0.00 sec)

• 按照部门号升序而雇员的工资降序排序

mysql> select ename,deptno,sal from emp order by deptno asc,sal desc;
+--------+--------+---------+
| ename  | deptno | sal     |
+--------+--------+---------+
| KING   |     10 | 5000.00 |
| CLARK  |     10 | 2450.00 |
| MILLER |     10 | 1300.00 |
| SCOTT  |     20 | 3000.00 |
| FORD   |     20 | 3000.00 |
| JONES  |     20 | 2975.00 |
| ADAMS  |     20 | 1100.00 |
| SMITH  |     20 |  800.00 |
| BLAKE  |     30 | 2850.00 |
| ALLEN  |     30 | 1600.00 |
| TURNER |     30 | 1500.00 |
| WARD   |     30 | 1250.00 |
| MARTIN |     30 | 1250.00 |
| JAMES  |     30 |  950.00 |
+--------+--------+---------+
14 rows in set (0.00 sec)

• 使用年薪进行降序排序

这里的年薪等于表中的 sal（工资）* 12 + comm （奖金） ，comm 可能为null,不能直接相加，所以需要用到 ifnull 函数，ifnull（comm,0）表示如果comm为空，则使用 0

mysql> select ename,sal*12+ifnull(comm,0) as '年薪' from emp order by 年薪 desc;
+--------+----------+
| ename  | 年薪     |
+--------+----------+
| KING   | 60000.00 |
| SCOTT  | 36000.00 |
| FORD   | 36000.00 |
| JONES  | 35700.00 |
| BLAKE  | 34200.00 |
| CLARK  | 29400.00 |
| ALLEN  | 19500.00 |
| TURNER | 18000.00 |
| MARTIN | 16400.00 |
| MILLER | 15600.00 |
| WARD   | 15500.00 |
| ADAMS  | 13200.00 |
| JAMES  | 11400.00 |
| SMITH  |  9600.00 |
+--------+----------+
14 rows in set (0.00 sec)

• 显示工资最高的员工的名字和工作岗位

mysql> select ename,job from emp where sal=(select max(sal) from emp);
+-------+-----------+
| ename | job       |
+-------+-----------+
| KING  | PRESIDENT |
+-------+-----------+
1 row in set (0.01 sec)

• 显示工资高于平均工资的员工信息

mysql> select ename,sal from emp where sal > (select avg(sal) from emp);
+-------+---------+
| ename | sal     |
+-------+---------+
| JONES | 2975.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| SCOTT | 3000.00 |
| KING  | 5000.00 |
| FORD  | 3000.00 |
+-------+---------+
6 rows in set (0.00 sec)

• 显示每个部门的平均工资和最高工资

这里是要求每个部门的平均工资和最高工资，则要用到分组查询，在每个部门内比较平均工资和最高工资。

mysql> select deptno, avg(sal),max(sal) from emp group by deptno;
+--------+-------------+----------+
| deptno | avg(sal)    | max(sal) |
+--------+-------------+----------+
|     10 | 2916.666667 |  5000.00 |
|     20 | 2175.000000 |  3000.00 |
|     30 | 1566.666667 |  2850.00 |
+--------+-------------+----------+
3 rows in set (0.00 sec)

• 显示平均工资低于2000的部门号和它的平均工资

这里和前一个案例一样，也是需要用到分组查询，分组查询后需要用 having 配合筛选

mysql> select deptno,avg(sal) as avg_sal from emp group by deptno having avg_sal < 2000;
+--------+-------------+
| deptno | avg_sal     |
+--------+-------------+
|     30 | 1566.666667 |
+--------+-------------+
1 row in set (0.00 sec)

• 显示每种岗位的雇员总数，平均工资

mysql> select job,count(*) as 人数 ,avg(sal) as 平均工资 from emp group by job;
+-----------+--------+--------------+
| job       | 人数   | 平均工资     |
+-----------+--------+--------------+
| ANALYST   |      2 |  3000.000000 |
| CLERK     |      4 |  1037.500000 |
| MANAGER   |      3 |  2758.333333 |
| PRESIDENT |      1 |  5000.000000 |
| SALESMAN  |      4 |  1400.000000 |
+-----------+--------+--------------+
5 rows in set (0.00 sec)

多表查询

实际开发中往往数据来自不同的表，所以需要多表查询。我们用一个简单的公司管理系统，有三张表EMP,DEPT,SALGRADE来演示如何进行多表查询。

案例：

• 显示雇员名、雇员工资以及所在部门的名字，因为上面的数据来自EMP和DEPT表，因此要联合查询。

mysql> select* from emp,dept;

上图是联合查询的结果，具体联合方式如下图：

所以一共会有 14 * 4 = 56 条数据，但是我们从图中看到，直接联合查询而不加筛选条件，表中的一些数据是没有意义的，比如下图SMITH对应的部门号应该只有20才对。

其实我们只要emp表中的deptno = dept表中的deptno字段的记录

mysql> select* from emp,dept where emp.deptno=dept.deptno;
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
| empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno | deptno | dname      | loc      |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |     30 | SALES      | CHICAGO  |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |     30 | SALES      | CHICAGO  |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     30 | SALES      | CHICAGO  |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
| 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
| 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
| 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
| 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |     30 | SALES      | CHICAGO  |
| 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
| 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
| 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
| 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
14 rows in set (0.00 sec)

• 显示部门号为10的部门名，员工名和工资

mysql> select ename,dname,sal from emp,dept where emp.deptno = dept.deptno and dept.deptno = 10;
+--------+------------+---------+
| ename  | dname      | sal     |
+--------+------------+---------+
| CLARK  | ACCOUNTING | 2450.00 |
| KING   | ACCOUNTING | 5000.00 |
| MILLER | ACCOUNTING | 1300.00 |
+--------+------------+---------+
3 rows in set (0.00 sec)

• 显示各个员工的姓名，工资，及工资级别

mysql> select ename,sal,grade from emp,salgrade where emp.sal between losal and hisal;
+--------+---------+-------+
| ename  | sal     | grade |
+--------+---------+-------+
| SMITH  |  800.00 |     1 |
| ALLEN  | 1600.00 |     3 |
| WARD   | 1250.00 |     2 |
| JONES  | 2975.00 |     4 |
| MARTIN | 1250.00 |     2 |
| BLAKE  | 2850.00 |     4 |
| CLARK  | 2450.00 |     4 |
| SCOTT  | 3000.00 |     4 |
| KING   | 5000.00 |     5 |
| TURNER | 1500.00 |     3 |
| ADAMS  | 1100.00 |     1 |
| JAMES  |  950.00 |     1 |
| FORD   | 3000.00 |     4 |
| MILLER | 1300.00 |     2 |
+--------+---------+-------+
14 rows in set (0.00 sec)

子连接

自连接是指在同一张表连接查询

案例：

显示员工 FORD 的上级领导的编号和姓名 （mgr 是员工领导的编号 empno）

• 使用子查询的方法：

mysql> select empno,ename from emp where emp.empno = (select mgr from emp where ename = 'FORD');
+--------+-------+
| empno  | ename |
+--------+-------+
| 007566 | JONES |
+--------+-------+
1 row in set (0.00 sec)

• 使用多表查询（自查询）

//使用到表的别名
//from emp leader, emp worker，给自己的表起别名，因为要先做笛卡尔积，所以别名可以先识别

mysql> select leader.empno,leader.ename from emp as leader,emp as worker where leader.empno=worker.mgr and worker.ename='FORD';
+--------+-------+
| empno  | ename |
+--------+-------+
| 007566 | JONES |
+--------+-------+
1 row in set (0.00 sec)

子查询

子查询是指嵌入在其他sql语句中的select语句，也叫嵌套查询

单行子查询

返回一行记录的子查询

• 显示SMITH同一部门的员工

mysql> select* from emp where deptno=(select deptno from emp where ename='SMITH');
+--------+-------+---------+------+---------------------+---------+------+--------+
| empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
+--------+-------+---------+------+---------------------+---------+------+--------+
| 007369 | SMITH | CLERK   | 7902 | 1980-12-17 00:00:00 |  800.00 | NULL |     20 |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL |     20 |
| 007876 | ADAMS | CLERK   | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL |     20 |
| 007902 | FORD  | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 |
+--------+-------+---------+------+---------------------+---------+------+--------+
5 rows in set (0.00 sec)

多行子查询

返回多行记录的子查询

• in 关键字：MySQL中的IN关键字用于判断某个字段的值是否在指定的集合中。如果字段的值在集合中，则满足条件，该字段所在的数据将会被查询出来。

案例：查询和10号部门的工作岗位相同的雇员的名字，岗位，工资，部门号，但是不包含10自己的

mysql> select ename,job,sal,deptno from emp where job in (select distinct job from emp where deptno=10) and deptno<>10;
+-------+---------+---------+--------+
| ename | job     | sal     | deptno |
+-------+---------+---------+--------+
| JONES | MANAGER | 2975.00 |     20 |
| BLAKE | MANAGER | 2850.00 |     30 |
| SMITH | CLERK   |  800.00 |     20 |
| ADAMS | CLERK   | 1100.00 |     20 |
| JAMES | CLERK   |  950.00 |     30 |
+-------+---------+---------+--------+
5 rows in set (0.00 sec)

• all 关键字：会列举出所有查询到的结果，通常与where使用，作比较。

案例：显示工资比部门30的所有员工的工资高的员工的姓名、工资和部门号

mysql> select ename,sal,deptno from emp where sal > all(select sal from emp where deptno=30);
+-------+---------+--------+
| ename | sal     | deptno |
+-------+---------+--------+
| JONES | 2975.00 |     20 |
| SCOTT | 3000.00 |     20 |
| KING  | 5000.00 |     10 |
| FORD  | 3000.00 |     20 |
+-------+---------+--------+
4 rows in set (0.00 sec)

• any 关键字：通常和where一起使用，any关键词可以理解为"对于子查询返回的列中的任一数值，如果比较结果为true，则返回true"。

案例：显示工资比部门30的任意员工的工资高的员工的姓名、工资和部门号（包含自己部门的员工）

mysql> select ename,sal,deptno from emp where sal > any(select sal from emp where deptno=30);
+--------+---------+--------+
| ename  | sal     | deptno |
+--------+---------+--------+
| ALLEN  | 1600.00 |     30 |
| WARD   | 1250.00 |     30 |
| JONES  | 2975.00 |     20 |
| MARTIN | 1250.00 |     30 |
| BLAKE  | 2850.00 |     30 |
| CLARK  | 2450.00 |     10 |
| SCOTT  | 3000.00 |     20 |
| KING   | 5000.00 |     10 |
| TURNER | 1500.00 |     30 |
| ADAMS  | 1100.00 |     20 |
| FORD   | 3000.00 |     20 |
| MILLER | 1300.00 |     10 |
+--------+---------+--------+
12 rows in set (0.00 sec)

多列子查询

单行子查询是指子查询只返回单列，单行数据；多行子查询是指返回单列多行数据，都是针对单列而言的，而多列子查询则是指查询返回多个列数据的子查询语句

案例：查询和SMITH的部门和岗位完全相同的所有雇员，不含SMITH本人

mysql> select ename from emp where (deptno,job)=(select deptno,job from emp where ename='SMITH') and ename <> 'SMITH';
+-------+
| ename |
+-------+
| ADAMS |
+-------+
1 row in set (0.00 sec)

在 from 子句中使用子查询

子查询语句出现在from子句中。这里要用到数据查询的技巧，把一个子查询当做一个临时表使用。

案例：

• 显示每个高于自己部门平均工资的员工的姓名、部门、工资、平均工资

//获取各个部门的平均工资，将其看作临时表
mysql> select ename,deptno,sal,format(asal,2) from emp,(select avg(sal) as asal,deptno as dt from emp group by deptno) as tmp 
    -> where emp.sal > tmp.asal and emp.deptno=tmp.dt;
+-------+--------+---------+----------------+
| ename | deptno | sal     | format(asal,2) |
+-------+--------+---------+----------------+
| KING  |     10 | 5000.00 | 2,916.67       |
| JONES |     20 | 2975.00 | 2,175.00       |
| SCOTT |     20 | 3000.00 | 2,175.00       |
| FORD  |     20 | 3000.00 | 2,175.00       |
| ALLEN |     30 | 1600.00 | 1,566.67       |
| BLAKE |     30 | 2850.00 | 1,566.67       |
+-------+--------+---------+----------------+
6 rows in set (0.00 sec)

• 查找每个部门工资最高的人的姓名、工资、部门、最高工资

mysql> select emp.ename,emp.sal,emp.deptno,ms from emp,(select max(sal) as ms,deptno from emp group by deptno) tmp
    -> where emp.deptno=tmp.deptno and emp.sal=tmp.ms;
+-------+---------+--------+---------+
| ename | sal     | deptno | ms      |
+-------+---------+--------+---------+
| BLAKE | 2850.00 |     30 | 2850.00 |
| SCOTT | 3000.00 |     20 | 3000.00 |
| KING  | 5000.00 |     10 | 5000.00 |
| FORD  | 3000.00 |     20 | 3000.00 |
+-------+---------+--------+---------+
4 rows in set (0.00 sec)

• 显示每个部门的信息（部门名，编号，地址）和人员数量

mysql> select dept.deptno,dname,mycnt,loc from dept,(select count(*) as mycnt,deptno from emp group by deptno) as tmp
    -> where dept.deptno=tmp.deptno;
+--------+------------+-------+----------+
| deptno | dname      | mycnt | loc      |
+--------+------------+-------+----------+
|     10 | ACCOUNTING |     3 | NEW YORK |
|     20 | RESEARCH   |     5 | DALLAS   |
|     30 | SALES      |     6 | CHICAGO  |
+--------+------------+-------+----------+
3 rows in set (0.00 sec)

合并查询

在实际应用中，为了合并多个select的执行结果，可以使用集合操作符 union，union all

• union : 该操作符用于取得两个结果集的并集。当使用该操作符时，会自动去掉结果集中的重复行。

案例：将工资大于2500或职位是MANAGER的人找出来

mysql> select ename,sal,job from emp where sal > 2500 union
    -> select ename,sal,job from emp where job='MANAGER';
+-------+---------+-----------+
| ename | sal     | job       |
+-------+---------+-----------+
| JONES | 2975.00 | MANAGER   |
| BLAKE | 2850.00 | MANAGER   |
| SCOTT | 3000.00 | ANALYST   |
| KING  | 5000.00 | PRESIDENT |
| FORD  | 3000.00 | ANALYST   |
| CLARK | 2450.00 | MANAGER   |
+-------+---------+-----------+
6 rows in set (0.00 sec)

• union all ：该操作符用于取得两个结果集的并集。当使用该操作符时，不会去掉结果集中的重复行。

mysql> select ename,sal,job from emp where sal > 2500 union all select ename,sal,job from emp where job='MANAGER';
+-------+---------+-----------+
| ename | sal     | job       |
+-------+---------+-----------+
| JONES | 2975.00 | MANAGER   |
| BLAKE | 2850.00 | MANAGER   |
| SCOTT | 3000.00 | ANALYST   |
| KING  | 5000.00 | PRESIDENT |
| FORD  | 3000.00 | ANALYST   |
| JONES | 2975.00 | MANAGER   |
| BLAKE | 2850.00 | MANAGER   |
| CLARK | 2450.00 | MANAGER   |
+-------+---------+-----------+
8 rows in set (0.00 sec)