【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���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  select deptno,avg(sal) as avg_sal from emp group by deptno having avg_sal   

                    • 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 deptno10;mysql> select ename,job,sal,deptno from emp where job in(select job from emp where deptno=10) and deptno10;+-------+---------+---------+--------+| 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 deptno30;+--------+---------+--------+| 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 deptno30;+--------+---------+--------+| 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)