MySQL：复合查询 | 汪钦的博客

✨✨作者主页：嶔某✨✨

✨✨所属专栏：MySql✨

准备工作，创建一个雇员信息表（来自oracle 9i的经典测试表）

EMP员工表
DEPT部门表
SALGRADE工资等级表

多表查询

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，所以要联合查询

mysql> select ename, sal, emp.deptno from emp, dept
     > where emp.deptno=dept.deptno;
+--------+---------+--------+
| ename  | sal     | deptno |
+--------+---------+--------+
| SMITH  |  800.00 |     20 |
| 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 |
| JAMES  |  950.00 |     30 |
| FORD   | 3000.00 |     20 |
| MILLER | 1300.00 |     10 |
+--------+---------+--------+
14 rows in set (0.01 sec)

显示部门号为10的部门，员工名和工资

mysql> select dname,ename,sal from emp,dept
     > where emp.deptno=dept.deptno and emp.deptno=10;
+------------+--------+---------+
| dname      | ename  | sal     |
+------------+--------+---------+
| ACCOUNTING | CLARK  | 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、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 (0.06 sec)

自连接

自连接是指在同一张表中连接查询

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：

mysql> select ename,empno from emp
     > where (select mgr from emp where ename='FORD')=empno;
+-------+--------+
| ename | empno  |
+-------+--------+
| JONES | 007566 |
+-------+--------+
1 row in set (0.00 sec)

使用多表查询（自查询）：

mysql> select leader.ename, leader.empno from emp leader,emp worker
     > where leader.empno=worker.mgr and worker.ename='FORD';
+-------+--------+
| ename | empno  |
+-------+--------+
| JONES | 007566 |
+-------+--------+
1 row in set (0.02 sec)

子查询

子查询是指嵌入在其他sql语句中的select语句，也叫嵌套语句

单行子查询

只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 (select deptno from emp where ename='SMITH')=deptno;
+--------+-------+---------+------+---------------------+---------+------+--------+
| 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：查询和10号部门的工作岗位相同的雇员的名字、岗位、工资、部门号但是不包括10自己的

mysql> select ename, job, deptno from emp
     > where job in (select job from emp where deptno=10)
     > and deptno<>10;
+-------+---------+--------+
| ename | job     | deptno |
+-------+---------+--------+
| SMITH | CLERK   |     20 |
| JONES | MANAGER |     20 |
| BLAKE | MANAGER |     30 |
| ADAMS | CLERK   |     20 |
| JAMES | CLERK   |     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、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 (0.01 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、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 (0.01 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

查询和SMITH的部门和岗位完全相同的所有雇员，不包含SMITH本人

mysql> select * from emp
     > where (deptno,job)=(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)

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。在MySql中，一切皆为表。

显示高于自己部门平均工资的员工的姓名、部门、工资、平均工资

mysql> select emp.ename, emp.deptno, emp.sal, format(avg_sal,2) from
     > emp, (select avg(sal) avg_sal, deptno dpt from emp group by deptno) tmp
     > where emp.sal>avg_sal and emp.deptno=tmp.dpt;
+-------+--------+---------+-------------------+
| ename | deptno | sal     | format(avg_sal,2) |
+-------+--------+---------+-------------------+
| FORD  |     20 | 3000.00 | 2,175.00          |
| SCOTT |     20 | 3000.00 | 2,175.00          |
| JONES |     20 | 2975.00 | 2,175.00          |
| BLAKE |     30 | 2850.00 | 1,566.67          |
| ALLEN |     30 | 1600.00 | 1,566.67          |
| KING  |     10 | 5000.00 | 2,916.67          |
+-------+--------+---------+-------------------+
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 ename, sal, emp.deptno, max_sal from
     > emp,(select deptno,max(sal) max_sal from emp group by deptno) tmp
     > where emp.deptno=tmp.deptno and max_sal=sal;
+-------+---------+--------+---------+
| ename | sal     | deptno | max_sal |
+-------+---------+--------+---------+
| 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

使用子查询

mysql> select dept.deptno, dname, cnt, loc from
     > dept, (select count(*) cnt,deptno from emp group by deptno) tmp
     > where dept.deptno=tmp.deptno;
+--------+------------+-----+----------+
| deptno | dname      | cnt | loc      |
+--------+------------+-----+----------+
|     10 | ACCOUNTING |   3 | NEW YORK |
|     20 | RESEARCH   |   5 | DALLAS   |
|     30 | SALES      |   6 | CHICAGO  |
+--------+------------+-----+----------+
3 rows in set (0.00 sec)

使用多表（复杂、不建议）

mysql> select dept.deptno, dept.dname, dept.loc, count(*)
     > from emp,dept where dept.deptno=emp.deptno
     > group by dept.deptno,dept.dname,dept.loc;
+--------+------------+----------+----------+
| deptno | dname      | loc      | count(*) |
+--------+------------+----------+----------+
|     20 | RESEARCH   | DALLAS   |        5 |
|     30 | SALES      | CHICAGO  |        6 |
|     10 | ACCOUNTING | NEW YORK |        3 |
+--------+------------+----------+----------+
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。

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';--去掉了重复记录
+-------+---------+-----------+
| 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   |
+-------+---------+-----------+

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。

将工资大于25000或职位是MANAGER的人找出来

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   |
+-------+---------+-----------+

内连接

内连接实际上就是利用where子句对两种表形成的笛卡尔积进行筛选，是再开发过程中使用得最多的连接查询

语法：

1	select 字段 from 表一 inner join 表二 on 连接条件 and 其他条件;

外连接

外连接分为左外连接和右外连接

左外连接

联合查询时，如果在左侧的表需要完全显示，我们就用左外连接

语法：

1	select 字段 from 表一 left join 表二 on 连接条件

右外连接

联合查询时，如果右侧的表需要完全显示，我们就右外连接

1	select 字段 from 表一 right join 表二 on 连接条件

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