【MySQL】MySQL复合查询--多表查询/自连接/子查询

文章目录

• 1.基本查询回顾
• 2.多表查询
• 3.自连接
• 4.子查询
• • 4.1单行子查询
  • 4.2多行子查询
  • 4.3多列子查询
  • 4.4在from子句中使用子查询
  • 4.5合并查询
  • • 4.5.1 union
    • 4.5.2 union all

1.基本查询回顾

表的内容如下：

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)  mysql> select * from dept; +--------+------------+----------+ | deptno | dname      | loc      | +--------+------------+----------+ |     10 | ACCOUNTING | NEW YORK | |     20 | RESEARCH   | DALLAS   | |     30 | SALES      | CHICAGO  | |     40 | OPERATIONS | BOSTON   | +--------+------------+----------+ 4 rows in set (0.00 sec)  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) 

• 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

// 使用模糊查询 select * from emp where (sal>500 or job='MANAGER') and ename like 'J%'; // 使用函数 select * from emp where (sal>500 or job='MANAGER') and substring(ename,1,1)='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)  mysql> select * from emp where (sal>500 or job='MANAGER') and substring(ename,1,1)='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

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

• 使用年薪进行降序排序

  年薪等于工资*12+奖金

  需要对奖金进行判断，如果奖金为null，则奖金为0

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

• 显示工资最高的员工的名字和工作岗位

  这里使用分组查询即可，先查出最高的工资，然后查询工资等于最高工资的员工的姓名和工作岗位

select ename,job from emp where sal = (select max(sal) from emp);  mysql> select ename,job from emp where sal = (select max(sal) from emp); +-------+-----------+ | ename | job       | +-------+-----------+ | KING  | PRESIDENT | +-------+-----------+ 1 row in set (0.00 sec) 

• 显示工资高于平均工资的员工信息

  这里使用分组查询即可

select ename,sal from emp where sal > (select avg(sal) from emp);  mysql> select ename,sal from emp where sal > (select avg(sal) from emp); +-------+---------+ | ename | sal     | +-------+---------+ | JONES | 2975.00 | | BLAKE | 2850.00 | | TEST  | 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

select deptno,avg(sal),max(sal) from emp group by deptno;  mysql> select deptno,avg(sal),max(sal) from emp group by deptno; +--------+-------------+----------+ | deptno | avg(sal)    | max(sal) | +--------+-------------+----------+ |     10 | 2425.000000 |  5000.00 | |     20 | 2175.000000 |  3000.00 | |     30 | 1690.000000 |  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

select deptno,avg(sal) as avg_sal from emp group by deptno having avg_sal < 2000;  mysql> select deptno,avg(sal) as avg_sal from emp group by deptno having avg_sal < 2000; +--------+-------------+ | deptno | avg_sal     | +--------+-------------+ |     30 | 1690.000000 | +--------+-------------+ 1 row in set (0.00 sec)  

• 显示每种岗位的雇员总数，平均工资

select job,count(*), avg(sal) from emp group by job;  mysql> select job,count(*), avg(sal) from emp group by job; +-----------+----------+-------------+ | job       | count(*) | avg(sal)    | +-----------+----------+-------------+ | 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) 

2.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。本节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

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

select ename,sal,dname from emp,dept where emp.deptno=dept.deptno; mysql> select ename,sal,dname from emp,dept where emp.deptno=dept.deptno; +--------+---------+------------+ | ename  | sal     | dname      | +--------+---------+------------+ | SMITH  |  800.00 | RESEARCH   | | ALLEN  | 1600.00 | SALES      | | WARD   | 1250.00 | SALES      | | JONES  | 2975.00 | RESEARCH   | | MARTIN | 1250.00 | SALES      | | BLAKE  | 2850.00 | SALES      | | CLACK  | 2450.00 | ACCOUNTING | | SCOTT  | 3000.00 | RESEARCH   | | KING   | 5000.00 | ACCOUNTING | | TURNER | 1500.00 | SALES      | | ADAMS  | 1100.00 | RESEARCH   | | JAMES  |  950.00 | SALES      | | FORD   | 3000.00 | RESEARCH   | | MILLER | 1300.00 | ACCOUNTING | +--------+---------+------------+ 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 dname,ename,sal from emp,dept where emp.deptno=dept.deptno and dept.deptno=10; +------------+--------+---------+ | dname      | ename  | sal     | +------------+--------+---------+ | ACCOUNTING | CLACK  | 2450.00 | | ACCOUNTING | KING   | 5000.00 | | ACCOUNTING | MILLER | 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 sal between losal and hisal; mysql> select ename,sal,grade from emp,salgrade where 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 | | CLACK  | 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)  

3.自连接

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）

使用的子查询

select ename,empno from emp where empno=(select mgr from emp where ename='FORD'); 

使用多表查询（自查询）

select e2.ename,e2.empno from emp e1,emp e2 where e1.ename='FORD' and e1.mgr=e2.empno; mysql> select e1.ename,e2.empno from emp e1,emp e2 where e1.ename='FORD' and e1.mgr=e2.empno; +-------+--------+ | ename | empno  | +-------+--------+ | FORD  | 007566 | +-------+--------+ 1 row in set (0.00 sec) 

4.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

4.1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

select * from emp where deptno=(select deptno from emp where ename='SMITH'); 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)  

4.2多行子查询

返回多行记录的子查询

• in关键字；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

  select ename,job,sal,deptno from emp where job in(select job from emp where deptno=10) and deptno<>10; mysql> select ename,job,sal,deptno from emp where job in(select 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；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

  // 使用聚合函数 select ename,sal,deptno from emp where sal>(select max(sal) from emp where deptno=30); mysql> select ename,sal,deptno from emp where sal>(select max(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 (0.01 sec)  // 使用all关键子 select ename,sal,deptno from emp where sal>all(select sal from emp where deptno=30); 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；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）

// 使用聚合函数 mysql> select ename,sal,deptno from emp where sal > (select min(sal) from emp where deptno=30) and deptno<>30; +--------+---------+--------+ | ename  | sal     | deptno | +--------+---------+--------+ | JONES  | 2975.00 |     20 | | CLACK  | 2450.00 |     10 | | SCOTT  | 3000.00 |     20 | | KING   | 5000.00 |     10 | | ADAMS  | 1100.00 |     20 | | FORD   | 3000.00 |     20 | | MILLER | 1300.00 |     10 | +--------+---------+--------+ 7 rows in set (0.00 sec) // 使用any关键字 mysql> select ename,sal,deptno from emp where sal > any(select sal from emp where deptno=30) and deptno<>30; +--------+---------+--------+ | ename  | sal     | deptno | +--------+---------+--------+ | JONES  | 2975.00 |     20 | | CLACK  | 2450.00 |     10 | | SCOTT  | 3000.00 |     20 | | KING   | 5000.00 |     10 | | ADAMS  | 1100.00 |     20 | | FORD   | 3000.00 |     20 | | MILLER | 1300.00 |     10 | +--------+---------+--------+ 7 rows in set (0.00 sec) 

4.3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 * from emp where (deptno,job)=(select deptno,job from emp where ename='SMITH') and ename<>'SMITH'; mysql> select * from emp where (deptno,job)in(select deptno,job from emp where ename='SMITH') and ename<>'SMITH'; +--------+-------+-------+------+---------------------+---------+------+--------+ | empno  | ename | job   | mgr  | hiredate            | sal     | comm | deptno | +--------+-------+-------+------+---------------------+---------+------+--------+ | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL |     20 | +--------+-------+-------+------+---------------------+---------+------+--------+ 1 row in set (0.00 sec)  

4.4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

答案：

select t1.ename,t1.deptno,t1.sal,t2.myavg from emp t1,(select deptno,avg(sal) myavg from emp group by deptno) t2 where t1.deptno=t2.deptno and t1.ssal > t2.myavg; 

步骤：

// 1.根据部门号分组得到每组的平均工资 mysql> select avg(sal) from emp group by deptno; +-------------+ | avg(sal)    | +-------------+ | 2916.666667 | | 2175.000000 | | 1566.666667 | +-------------+ 3 rows in set (0.00 sec)  // 2.根据部门号分组得到每组的平均工资和部门号 mysql> select deptno,avg(sal) from emp group by deptno; +--------+-------------+ | deptno | avg(sal)    | +--------+-------------+ |     10 | 2916.666667 | |     20 | 2175.000000 | |     30 | 1566.666667 | +--------+-------------+ 3 rows in set (0.00 sec)  // 3.将上面得到的结果与emp表做笛卡尔积 mysql> select * from emp t1,(select deptno,avg(sal) myavg from emp group by deptno) t2 where t1.deptno=t2.deptno; +--------+--------+-----------+------+---------------------+---------+---------+--------+--------+-------------+ | empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno | deptno | myavg       | +--------+--------+-----------+------+---------------------+---------+---------+--------+--------+-------------+ | 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |     20 | 2175.000000 | | 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |     30 | 1566.666667 | | 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |     30 | 1566.666667 | | 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |     20 | 2175.000000 | | 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     30 | 1566.666667 | | 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |     30 | 1566.666667 | | 007782 | CLACK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |     10 | 2916.666667 | | 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |     20 | 2175.000000 | | 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |     10 | 2916.666667 | | 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |     30 | 1566.666667 | | 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |     20 | 2175.000000 | | 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |     30 | 1566.666667 | | 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |     20 | 2175.000000 | | 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |     10 | 2916.666667 | +--------+--------+-----------+------+---------------------+---------+---------+--------+--------+-------------+ 14 rows in set (0.00 sec)  // 5.增加筛选条件 :工资大于平均工资 mysql> select * from emp t1,(select deptno,avg(sal) myavg from emp group by deptno) t2 where t1.deptno=t2.deptno and t1.sal > t2.myavg; +--------+-------+-----------+------+---------------------+---------+--------+--------+--------+-------------+ | empno  | ename | job       | mgr  | hiredate            | sal     | comm   | deptno | deptno | myavg       | +--------+-------+-----------+------+---------------------+---------+--------+--------+--------+-------------+ | 007499 | ALLEN | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 |     30 |     30 | 1566.666667 | | 007566 | JONES | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |   NULL |     20 |     20 | 2175.000000 | | 007698 | BLAKE | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |   NULL |     30 |     30 | 1566.666667 | | 007788 | SCOTT | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |   NULL |     20 |     20 | 2175.000000 | | 007839 | KING  | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |   NULL |     10 |     10 | 2916.666667 | | 007902 | FORD  | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |   NULL |     20 |     20 | 2175.000000 | +--------+-------+-----------+------+---------------------+---------+--------+--------+--------+-------------+ 6 rows in set (0.00 sec)  // 5.根据题目要求得到结果 mysql> select t1.ename,t1.deptno,t1.sal,t2.myavg from emp t1,(select deptno,avg(sal) myavg from emp group by deptno) t2 where t1.deptno=t2.deptno and t1.ssal > t2.myavg; +-------+--------+---------+-------------+ | ename | deptno | sal     | myavg       | +-------+--------+---------+-------------+ | ALLEN |     30 | 1600.00 | 1566.666667 | | JONES |     20 | 2975.00 | 2175.000000 | | BLAKE |     30 | 2850.00 | 1566.666667 | | SCOTT |     20 | 3000.00 | 2175.000000 | | KING  |     10 | 5000.00 | 2916.666667 | | FORD  |     20 | 3000.00 | 2175.000000 | +-------+--------+---------+-------------+ 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

答案：

select t1.ename,t1.sal,t1.deptno,t2.mymax from emp t1,(select deptno, max(sal) mymax from emp group by deptno) t2 where t1.deptno=t2.deptno and t1..sal=t2.mymax; 

步骤：

// 1.得到分组之后的部门号和最高工资 mysql> select deptno, max(sal) from emp group by deptno; +--------+----------+ | deptno | max(sal) | +--------+----------+ |     10 |  5000.00 | |     20 |  3000.00 | |     30 |  2850.00 | +--------+----------+ 3 rows in set (0.01 sec)  // 2.与emp表进行笛卡尔积并进行t1.sal=t2.mymax的筛选(工资等于最高工资) mysql> select * from emp t1,(select deptno, max(sal) mymax from emp group by deptno) t2 where t1.deptno=t2.deptno and t1.sal=t2.mymax; +--------+-------+-----------+------+---------------------+---------+------+--------+--------+---------+ | empno  | ename | job       | mgr  | hiredate            | sal     | comm | deptno | deptno | mymax   | +--------+-------+-----------+------+---------------------+---------+------+--------+--------+---------+ | 007698 | BLAKE | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 |     30 | 2850.00 | | 007788 | SCOTT | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL |     20 |     20 | 3000.00 | | 007839 | KING  | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL |     10 |     10 | 5000.00 | | 007902 | FORD  | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 |     20 | 3000.00 | +--------+-------+-----------+------+---------------------+---------+------+--------+--------+---------+ 4 rows in set (0.00 sec)  // 3.根据题目要求选择需要筛选的内容 mysql> select t1.ename,t1.sal,t1.deptno,t2.mymax from emp t1,(select deptno, max(sal) mymax from emp group by deptno) t2 where t1.deptno=t2.deptno and t1..sal=t2.mymax; +-------+---------+--------+---------+ | ename | sal     | deptno | mymax   | +-------+---------+--------+---------+ | 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

答案：

select t1.deptno,t1.dname,t1.loc,t2.num from dept t1,(select deptno,count(*) num from emp group by deptno) t2 where t1.deptno=t2.deptno; 

步骤：

// 1.分组得到每一组的人数 mysql> select deptno,count(*) num from emp group by deptno; +--------+-----+ | deptno | num | +--------+-----+ |     10 |   3 | |     20 |   5 | |     30 |   6 | +--------+-----+ 3 rows in set (0.00 sec)  // 2.和部门表进行笛卡尔积，然后进行条件筛选 mysql> select * from dept t1,(select deptno,count(*) num from emp group by deptno) t2 where t1.deptno=t2.deptno; +--------+------------+----------+--------+-----+ | deptno | dname      | loc      | deptno | num | +--------+------------+----------+--------+-----+ |     10 | ACCOUNTING | NEW YORK |     10 |   3 | |     20 | RESEARCH   | DALLAS   |     20 |   5 | |     30 | SALES      | CHICAGO  |     30 |   6 | +--------+------------+----------+--------+-----+ 3 rows in set (0.01 sec) mysql> select t1.deptno,t1.dname,t1.loc,t2.num from dept t1,(select deptno,count(*) num from emp group by deptno) t2 where t1.deptno=t2.deptno; +--------+------------+----------+-----+ | deptno | dname      | loc      | num | +--------+------------+----------+-----+ |     10 | ACCOUNTING | NEW YORK |   3 | |     20 | RESEARCH   | DALLAS   |   5 | |     30 | SALES      | CHICAGO  |   6 | +--------+------------+----------+-----+ 3 rows in set (0.00 sec)  

暴力解法：

mysql> select dept.dname,dept.deptno,dept.loc,count(*) from emp,dept where emp.deptno=dept.deptno group by dept.deptno,dept.dname,dept.loc; +------------+--------+----------+----------+ | dname      | deptno | loc      | count(*) | +------------+--------+----------+----------+ | ACCOUNTING |     10 | NEW YORK |        3 | | RESEARCH   |     20 | DALLAS   |        5 | | SALES      |     30 | CHICAGO  |        6 | +------------+--------+----------+----------+ 3 rows in set (0.01 sec) 

总结：

解决多表问题的本质：想办法将多表转化为单表，所以mysql中，所有select的问题全部都可以转化成单表问题

4.5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

4.5.1 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

// 1.查出工资大于2500的 mysql> select * from emp where sal>2500; +--------+-------+-----------+------+---------------------+---------+------+--------+ | empno  | ename | job       | mgr  | hiredate            | sal     | comm | deptno | +--------+-------+-----------+------+---------------------+---------+------+--------+ | 007566 | JONES | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 | | 007698 | BLAKE | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 | | 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 | | 007902 | FORD  | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 | +--------+-------+-----------+------+---------------------+---------+------+--------+ 5 rows in set (0.00 sec)  // 2.查出job=MANAGER的 mysql> select * from emp where job='MANAGER'; +--------+-------+---------+------+---------------------+---------+------+--------+ | empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno | +--------+-------+---------+------+---------------------+---------+------+--------+ | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 | | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 | | 007782 | CLACK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL |     10 | +--------+-------+---------+------+---------------------+---------+------+--------+ 3 rows in set (0.00 sec)  // 3.进行合并 mysql> select * from emp where sal>2500 union select * from emp where job='MANAGER'; +-------+-------+-----------+------+---------------------+---------+------+--------+ | empno | ename | job       | mgr  | hiredate            | sal     | comm | deptno | +-------+-------+-----------+------+---------------------+---------+------+--------+ |  7566 | JONES | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 | |  7698 | BLAKE | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 | |  7788 | SCOTT | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL |     20 | |  7839 | KING  | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL |     10 | |  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 | |  7782 | CLACK | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL |     10 | +-------+-------+-----------+------+---------------------+---------+------+--------+ 6 rows in set (0.00 sec) 

4.5.2 union all

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。

案例：将工资大于25000或职位是MANAGER的人找出来

// 1.查出工资大于2500的 mysql> select * from emp where sal>2500; +--------+-------+-----------+------+---------------------+---------+------+--------+ | empno  | ename | job       | mgr  | hiredate            | sal     | comm | deptno | +--------+-------+-----------+------+---------------------+---------+------+--------+ | 007566 | JONES | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 | | 007698 | BLAKE | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 | | 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 | | 007902 | FORD  | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 | +--------+-------+-----------+------+---------------------+---------+------+--------+ 5 rows in set (0.00 sec)  // 2.查出job=MANAGER的 mysql> select * from emp where job='MANAGER'; +--------+-------+---------+------+---------------------+---------+------+--------+ | empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno | +--------+-------+---------+------+---------------------+---------+------+--------+ | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 | | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 | | 007782 | CLACK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL |     10 | +--------+-------+---------+------+---------------------+---------+------+--------+ 3 rows in set (0.01 sec)  // 3.进行合并 mysql> select * from emp where sal>2500 union all select * from emp where job='MANAGER'; +-------+-------+-----------+------+---------------------+---------+------+--------+ | empno | ename | job       | mgr  | hiredate            | sal     | comm | deptno | +-------+-------+-----------+------+---------------------+---------+------+--------+ |  7566 | JONES | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 | |  7698 | BLAKE | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 | |  7788 | SCOTT | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL |     20 | |  7839 | KING  | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL |     10 | |  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 | |  7566 | JONES | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 | |  7698 | BLAKE | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 | |  7782 | CLACK | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL |     10 | +-------+-------+-----------+------+---------------------+---------+------+--------+ 8 rows in set (0.00 sec)