【MySQL学习笔记】（八）复合查询
在前面的笔记中做的查询基本都是对一张表进行查询，在实际开发中远远不够，本篇文章内容是复合查询相关的笔记。需要用到oracle9i的经典测试表，在笔记（六）中已经教大家如何导入了。
复合查询
- 基本查询回顾
- 多表查询
- 子连接
- 子查询
-
- 单行子查询
- 多行子查询
- 多列子查询
- 在 from 子句中使用子查询
- 合并查询
基本查询回顾
这是 emp 表
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)
- 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
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)
- 按照部门号升序而雇员的工资降序排序
mysql> select ename,deptno,sal from emp order by deptno asc,sal desc;
+--------+--------+---------+
| ename | deptno | sal |
+--------+--------+---------+
| KING | 10 | 5000.00 |
| CLARK | 10 | 2450.00 |
| MILLER | 10 | 1300.00 |
| SCOTT | 20 | 3000.00 |
| FORD | 20 | 3000.00 |
| JONES | 20 | 2975.00 |
| ADAMS | 20 | 1100.00 |
| SMITH | 20 | 800.00 |
| BLAKE | 30 | 2850.00 |
| ALLEN | 30 | 1600.00 |
| TURNER | 30 | 1500.00 |
| WARD | 30 | 1250.00 |
| MARTIN | 30 | 1250.00 |
| JAMES | 30 | 950.00 |
+--------+--------+---------+
14 rows in set (0.00 sec)
- 使用年薪进行降序排序
这里的年薪等于表中的 sal（工资）* 12 + comm （奖金） ，comm 可能为null,不能直接相加，所以需要用到 ifnull 函数，ifnull（comm,0）表示如果comm为空，则使用 0
mysql> select ename,sal*12+ifnull(comm,0) as '年薪' from emp order by 年薪 desc;
+--------+----------+
| ename | 年薪 |
+--------+----------+
| KING | 60000.00 |
| SCOTT | 36000.00 |
| FORD | 36000.00 |
| JONES | 35700.00 |
| BLAKE | 34200.00 |
| CLARK | 29400.00 |
| ALLEN | 19500.00 |
| TURNER | 18000.00 |
| MARTIN | 16400.00 |
| MILLER | 15600.00 |
| WARD | 15500.00 |
| ADAMS | 13200.00 |
| JAMES | 11400.00 |
| SMITH | 9600.00 |
+--------+----------+
14 rows in set (0.00 sec)
- 显示工资最高的员工的名字和工作岗位
mysql> select ename,job from emp where sal=(select max(sal) from emp);
+-------+-----------+
| ename | job |
+-------+-----------+
| KING | PRESIDENT |
+-------+-----------+
1 row in set (0.01 sec)
- 显示工资高于平均工资的员工信息
mysql> select ename,sal from emp where sal > (select avg(sal) from emp);
+-------+---------+
| ename | sal |
+-------+---------+
| JONES | 2975.00 |
| BLAKE | 2850.00 |
| CLARK | 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
这里是要求每个部门的平均工资和最高工资，则要用到分组查询，在每个部门内比较平均工资和最高工资。
mysql> select deptno, avg(sal),max(sal) from emp group by deptno;
+--------+-------------+----------+
| deptno | avg(sal) | max(sal) |
+--------+-------------+----------+
| 10 | 2916.666667 | 5000.00 |
| 20 | 2175.000000 | 3000.00 |
| 30 | 1566.666667 | 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
这里和前一个案例一样，也是需要用到分组查询，分组查询后需要用 having 配合筛选
mysql> select deptno,avg(sal) as avg_sal from emp group by deptno having avg_sal < 2000;
+--------+-------------+
| deptno | avg_sal |
+--------+-------------+
| 30 | 1566.666667 |
+--------+-------------+
1 row in set (0.00 sec)
- 显示每种岗位的雇员总数，平均工资
mysql> select job,count(*) as 人数 ,avg(sal) as 平均工资 from emp group by job;
+-----------+--------+--------------+
| job | 人数 | 平均工资 |
+-----------+--------+--------------+
| 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)
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。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。
mysql> select* from emp,dept;
上图是联合查询的结果，具体联合方式如下图：
所以一共会有 14 * 4 = 56 条数据，但是我们从图中看到，直接联合查询而不加筛选条件，表中的一些数据是没有意义的，比如下图SMITH对应的部门号应该只有20才对。
其实我们只要emp表中的deptno = dept表中的deptno字段的记录
mysql> select* from emp,dept where emp.deptno=dept.deptno;
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno | deptno | dname | loc |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 30 | SALES | CHICAGO |
| 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 30 | SALES | CHICAGO |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 30 | SALES | CHICAGO |
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 30 | SALES | CHICAGO |
| 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK |
| 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 30 | SALES | CHICAGO |
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 30 | SALES | CHICAGO |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
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 ename,dname,sal from emp,dept where emp.deptno = dept.deptno and dept.deptno = 10;
+--------+------------+---------+
| ename | dname | sal |
+--------+------------+---------+
| CLARK | ACCOUNTING | 2450.00 |
| KING | ACCOUNTING | 5000.00 |
| MILLER | ACCOUNTING | 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 emp.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 |
| CLARK | 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)
子连接
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）
- 使用子查询的方法：
mysql> select empno,ename from emp where emp.empno = (select mgr from emp where ename = 'FORD');
+--------+-------+
| empno | ename |
+--------+-------+
| 007566 | JONES |
+--------+-------+
1 row in set (0.00 sec)
- 使用多表查询（自查询）
//使用到表的别名
//from emp leader, emp worker，给自己的表起别名，因为要先做笛卡尔积，所以别名可以先识别
mysql> select leader.empno,leader.ename from emp as leader,emp as worker where leader.empno=worker.mgr and worker.ename='FORD';
+--------+-------+
| empno | ename |
+--------+-------+
| 007566 | JONES |
+--------+-------+
1 row in set (0.00 sec)
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
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
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)
多行子查询
返回多行记录的子查询
- in 关键字：MySQL中的IN关键字用于判断某个字段的值是否在指定的集合中。如果字段的值在集合中，则满足条件，该字段所在的数据将会被查询出来。
案例：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
mysql> select ename,job,sal,deptno from emp where job in (select distinct 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：会列举出所有查询到的结果，通常与where使用，作比较。
案例：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
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：通常和where一起使用，any关键词可以理解为“对于子查询返回的列中的任一数值，如果比较结果为true，则返回true”。
案例：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）
mysql> select ename,sal,deptno from emp where sal > any(select sal from emp where deptno=30);
+--------+---------+--------+
| ename | sal | deptno |
+--------+---------+--------+
| ALLEN | 1600.00 | 30 |
| WARD | 1250.00 | 30 |
| JONES | 2975.00 | 20 |
| MARTIN | 1250.00 | 30 |
| BLAKE | 2850.00 | 30 |
| CLARK | 2450.00 | 10 |
| SCOTT | 3000.00 | 20 |
| KING | 5000.00 | 10 |
| TURNER | 1500.00 | 30 |
| ADAMS | 1100.00 | 20 |
| FORD | 3000.00 | 20 |
| MILLER | 1300.00 | 10 |
+--------+---------+--------+
12 rows in set (0.00 sec)
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 ename from emp where (deptno,job)=(select deptno,job from emp where ename='SMITH') and ename <> 'SMITH';
+-------+
| ename |
+-------+
| ADAMS |
+-------+
1 row in set (0.00 sec)
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
//获取各个部门的平均工资，将其看作临时表
mysql> select ename,deptno,sal,format(asal,2) from emp,(select avg(sal) as asal,deptno as dt from emp group by deptno) as tmp
-> where emp.sal > tmp.asal and emp.deptno=tmp.dt;
+-------+--------+---------+----------------+
| ename | deptno | sal | format(asal,2) |
+-------+--------+---------+----------------+
| KING | 10 | 5000.00 | 2,916.67 |
| JONES | 20 | 2975.00 | 2,175.00 |
| SCOTT | 20 | 3000.00 | 2,175.00 |
| FORD | 20 | 3000.00 | 2,175.00 |
| ALLEN | 30 | 1600.00 | 1,566.67 |
| BLAKE | 30 | 2850.00 | 1,566.67 |
+-------+--------+---------+----------------+
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
mysql> select emp.ename,emp.sal,emp.deptno,ms from emp,(select max(sal) as ms,deptno from emp group by deptno) tmp
-> where emp.deptno=tmp.deptno and emp.sal=tmp.ms;
+-------+---------+--------+---------+
| ename | sal | deptno | ms |
+-------+---------+--------+---------+
| 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
mysql> select dept.deptno,dname,mycnt,loc from dept,(select count(*) as mycnt,deptno from emp group by deptno) as tmp
-> where dept.deptno=tmp.deptno;
+--------+------------+-------+----------+
| deptno | dname | mycnt | loc |
+--------+------------+-------+----------+
| 10 | ACCOUNTING | 3 | NEW YORK |
| 20 | RESEARCH | 5 | DALLAS |
| 30 | SALES | 6 | CHICAGO |
+--------+------------+-------+----------+
3 rows in set (0.00 sec)
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
- 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的人找出来
mysql> select ename,sal,job from emp where sal > 2500 union
-> select ename,sal,job from emp where job='MANAGER';
+-------+---------+-----------+
| ename | sal | job |
+-------+---------+-----------+
| JONES | 2975.00 | MANAGER |
| BLAKE | 2850.00 | MANAGER |
| SCOTT | 3000.00 | ANALYST |
| KING | 5000.00 | PRESIDENT |
| FORD | 3000.00 | ANALYST |
| CLARK | 2450.00 | MANAGER |
+-------+---------+-----------+
6 rows in set (0.00 sec)
- union all ：该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。
mysql> select ename,sal,job from emp where sal > 2500 union all select ename,sal,job from emp where job='MANAGER';
+-------+---------+-----------+
| ename | sal | job |
+-------+---------+-----------+
| JONES | 2975.00 | MANAGER |
| BLAKE | 2850.00 | MANAGER |
| SCOTT | 3000.00 | ANALYST |
| KING | 5000.00 | PRESIDENT |
| FORD | 3000.00 | ANALYST |
| JONES | 2975.00 | MANAGER |
| BLAKE | 2850.00 | MANAGER |
| CLARK | 2450.00 | MANAGER |
+-------+---------+-----------+
8 rows in set (0.00 sec)