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)