MySQL复合查询

目录

前言

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

5.合并查询

总结

---

前言

        hello,各位小伙伴，大家好，很高兴，又和大家见面了！今天我们要介绍的内容是复合查询，在这之前介绍的都是对一张表的操作，但是在实际开发中，对一张表的操作是满足不了需求的，一般需要对多张表进行操作。而复合查询就为围绕对对多张表的操作进行介绍！

1.基本查询回顾

        在学习复合查询之前，我们首先来对之前学习过的基本查询做一个综合性的回顾和练习，然后再来看看复合查询的操作，下面我们就通过具体的案例进行复习。

a.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 ename,job,sal from emp where (sal > 500 or job='MANGER') and ename like 'J%'
    -> ;
+-------+---------+---------+
| ename | job     | sal     |
+-------+---------+---------+
| JONES | MANAGER | 2975.00 |
| JAMES | CLERK   |  950.00 |
+-------+---------+---------+
2 rows in set (0.00 sec)

b.按照部门号升序而雇员的工资降序排序

mysql> select* from emp order by deptno,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 | CLARK  | 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 |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
14 rows in set (0.01 sec)

c.使用年薪进行降序排序

mysql> select ename,sal*12+ifnull(comm,0) 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)

d.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 (0.00 sec)

e.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)

g.显示每个部门的平均工资和最高工资

mysql> select deptno,format(avg(sal),2),max(sal) from emp group by deptno;
+--------+--------------------+----------+
| deptno | format(avg(sal),2) | max(sal) |
+--------+--------------------+----------+
|     10 | 2,916.67           |  5000.00 |
|     20 | 2,175.00           |  3000.00 |
|     30 | 1,566.67           |  2850.00 |
+--------+--------------------+----------+
3 rows in set (0.00 sec)

h.显示平均工资低于2000的部门号和它的平均工资

mysql> select deptno,avg(sal) 平均工资 from emp group by deptno having 平均工资 < 2000;
+--------+--------------+
| deptno | 平均工资     |
+--------+--------------+
|     30 |  1566.666667 |
+--------+--------------+
1 row in set (0.00 sec)

i.显示每种岗位的雇员总数，平均工资

mysql> select job,count(*),avg(sal) 平均工资 from emp group by job;
+-----------+----------+--------------+
| job       | count(*) | 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)

以上就是我们通过几个具体的案例对之前学过的单表查询操作做了简单回顾，下面我们就对多表查询的操作进行介绍：

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：

a.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

联合查询的理解：如下表所示

许多数据都是无效的，我们只需要emp表中的deptno = dept表中的deptno字段的记录：

mysql> select emp.ename,emp.sal,dept.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      |
| CLARK  | 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)

b.显示部门号为10的部门名，员工名和工资

mysql> select emp.ename,emp.sal,dept.dname from emp,dept where emp.deptno = dept.deptno and dept.deptno = 10;
+--------+---------+------------+
| ename  | sal     | dname      |
+--------+---------+------------+
| CLARK  | 2450.00 | ACCOUNTING |
| KING   | 5000.00 | ACCOUNTING |
| MILLER | 1300.00 | ACCOUNTING |
+--------+---------+------------+
3 rows in set (0.00 sec)

c.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)

3.自连接

前面是我们是在不同的表中进行查询，而自连接是指在同一张表连接查询(本质是将多张表合并成一张表，然后在一张表中进行查询)；

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 leader,emp worker where leader.empno=worker.mgr and worker.ename='FORD';
+--------+-------+
| empno  | ename |
+--------+-------+
| 007566 | JONES |
+--------+-------+
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

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

4.2多行子查询

返回多行记录的子查询；

案例：

使用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

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；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 |
+-------+---------+--------+

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）

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)

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 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)

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

mysql> select ename,deptno,sal,平均工资 from emp,(select avg(sal) 平均工资,deptno dt from emp group by deptno) tmp where emp.sal > tmp.平均工资 an
nd emp.deptno=tmp.dt;
+-------+--------+---------+--------------+
| ename | deptno | sal     | 平均工资     |
+-------+--------+---------+--------------+
| KING  |     10 | 5000.00 |  2916.666667 |
| JONES |     20 | 2975.00 |  2175.000000 |
| SCOTT |     20 | 3000.00 |  2175.000000 |
| FORD  |     20 | 3000.00 |  2175.000000 |
| ALLEN |     30 | 1600.00 |  1566.666667 |
| BLAKE |     30 | 2850.00 |  1566.666667 |
+-------+--------+---------+--------------+

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,最高工资 from emp,(select max(sal) 最高工资 ,deptno from emp group by deptno) tmp where emp.deptno=tmp..deptno and emp.sal=tmp.最高工资;
+-------+---------+--------+--------------+
| ename | sal     | deptno | 最高工资     |
+-------+---------+--------+--------------+
| 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

方法1 ：使用多表

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

方法2 ： 使用子查询

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

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

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的人找出来

mysql> select ename,sal,job from emp where sal > 2500 union select ename,sal,job from emp where ename='MANAGER';
+-------+---------+-----------+
| ename | sal     | job       |
+-------+---------+-----------+
| JONES | 2975.00 | MANAGER   |
| BLAKE | 2850.00 | MANAGER   |
| SCOTT | 3000.00 | ANALYST   |
| KING  | 5000.00 | PRESIDENT |
| FORD  | 3000.00 | ANALYST   |
+-------+---------+-----------+
5 rows in set (0.00 sec)

2.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。

案例：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   |
+-------+---------+-----------+
8 rows in set (0.00 sec)

总结

        本篇文章为大家介绍了多表查询的几种方式，掌握了这些多表查询的方式之后，可以满足更多实际开发中的需求！