union union all 基本查询 【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 union4.5.2 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)

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