阅读量:2
文章目录
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) 