【MySQL】复合查询

文章目录

• 👉基本查询回顾👈
• • [select 子查询](#select 子查询)
• 👉多表查询👈
• 👉自连接👈
• 👉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
• 👉总结👈

👉基本查询回顾👈

• 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 = 'MANAGER') and (substring(ename, 1, 1) = 'J');
+-------+---------+---------+
| ename | job     | sal     |
+-------+---------+---------+
| JONES | MANAGER | 2975.00 |
| JAMES | CLERK   |  950.00 |
+-------+---------+---------+
2 rows in set (0.00 sec)

mysql> select ename, job, sal from emp where (sal > 500 or job = 'MANAGER') and (ename like 'J%');
+-------+---------+---------+
| ename | job     | sal     |
+-------+---------+---------+
| JONES | MANAGER | 2975.00 |
| JAMES | CLERK   |  950.00 |
+-------+---------+---------+
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 deptno, ename, sal from emp order by deptno asc, sal desc;
+--------+--------+---------+
| deptno | ename  | sal     |
+--------+--------+---------+
|     10 | KING   | 5000.00 |
|     10 | CLARK  | 2450.00 |
|     10 | MILLER | 1300.00 |
|     20 | SCOTT  | 3000.00 |
|     20 | FORD   | 3000.00 |
|     20 | JONES  | 2975.00 |
|     20 | ADAMS  | 1100.00 |
|     20 | SMITH  |  800.00 |
|     30 | BLAKE  | 2850.00 |
|     30 | ALLEN  | 1600.00 |
|     30 | TURNER | 1500.00 |
|     30 | WARD   | 1250.00 |
|     30 | MARTIN | 1250.00 |
|     30 | JAMES  |  950.00 |
+--------+--------+---------+
14 rows in set (0.00 sec)

• 使用年薪进行降序排序

-- 年薪=月薪*12+奖金
-- 如果奖金为null,则认为奖金为零
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

-- select ename, max(sal) from emp; 这条SQL语句
-- 是无符合语法规范的,因为并没有按照ename进行分组
-- 只有出现在group by中的列才能出现在select中

-- 这显然不是我们想要的结果
mysql> select ename, max(sal) from emp group by ename;
+--------+----------+
| ename  | max(sal) |
+--------+----------+
| ADAMS  |  1100.00 |
| ALLEN  |  1600.00 |
| BLAKE  |  2850.00 |
| CLARK  |  2450.00 |
| FORD   |  3000.00 |
| JAMES  |   950.00 |
| JONES  |  2975.00 |
| KING   |  5000.00 |
| MARTIN |  1250.00 |
| MILLER |  1300.00 |
| SCOTT  |  3000.00 |
| SMITH  |   800.00 |
| TURNER |  1500.00 |
| WARD   |  1250.00 |
+--------+----------+
14 rows in set (0.00 sec)

-- 可以先找出最高工资,然后就能将拥有最高工资的人名显示出来
mysql> select max(sal) from emp;
+----------+
| max(sal) |
+----------+
|  5000.00 |
+----------+
1 row in set (0.00 sec)

mysql> select ename, sal 最高工资 from emp where sal = 5000;
+-------+--------------+
| ename | 最高工资     |
+-------+--------------+
| KING  |      5000.00 |
+-------+--------------+
1 row in set (0.00 sec)

-- 上面用了两条SQL语句才完成需求,那么可不可以用
-- 一条SQL语句完成需求呢？当然可以
-- 通过嵌套select子查询
mysql> select ename, job from emp where sal = (select max(sal) from emp);
+-------+-----------+
| ename | job       |
+-------+-----------+
| KING  | PRESIDENT |
+-------+-----------+
1 row in set (0.00 sec)

select 子查询

• 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

mysql> select deptno, format(avg(sal), 2) 平均工资, max(sal) 最高工资 from emp group by deptno;
+--------+--------------+--------------+
| deptno | 平均工资     | 最高工资     |
+--------+--------------+--------------+
|     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)

• 显示平均工资低于 2000 的部门号和它的平均工资

-- format会对数据进行格式化,无法再进行与原数据的比较
-- fromat会用不同的编码来呈现原始数据
mysql> select charset(format(1000, 2));
+--------------------------+
| charset(format(1000, 2)) |
+--------------------------+
| utf8                     |
+--------------------------+
1 row in set (0.00 sec)

mysql> select charset(1000);
+---------------+
| charset(1000) |
+---------------+
| binary        |
+---------------+
1 row in set (0.00 sec)

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)

• 显示每种岗位的雇员总数，平均工资

mysql> select job 岗位, count(*) 人数, format(avg(sal), 2) 平均工资 from emp group by job;
+-----------+--------+--------------+
| 岗位      | 人数   | 平均工资     |
+-----------+--------+--------------+
| ANALYST   |      2 | 3,000.00     |
| CLERK     |      4 | 1,037.50     |
| MANAGER   |      3 | 2,758.33     |
| PRESIDENT |      1 | 5,000.00     |
| SALESMAN  |      4 | 1,400.00     |
+-----------+--------+--------------+
5 rows in set (0.00 sec)

👉多表查询👈

实际开发中数据往往来自不同的表，所以需要多表查询。这部分我们用一个简单的公司管理系统，有三张表 emp、dept 和 salgrade 来演示如何进行多表查询。

笛卡尔积

笛卡尔积是一种关系型数据库中的操作，用于将两个表的每一行进行组合，生成一个新的结果集。它将表之间的每个行组合，生成的结果集的行数等于两个表的行数的乘积。

假设有两个表 A 和 B，表 A 有 m 行，表 B 有 n 行，那么它们的笛卡尔积将生成一个包含 m * n 行的结果集。每个结果集的行包含来自表 A 和表 B 的一行组合。

笛卡尔积的计算方式是对两个表进行无条件的连接，即将表 A 的每一行与表 B 的每一行进行组合。结果集中的每一行都代表了表 A 和表 B 中一行的组合。

需要注意的是，笛卡尔积是对两个表进行无条件的连接的，所以新形成的表中的数据可能是一些无效的数据。因此在将两张表形成一张新表时，我们需要设置一些条件来避免数据无效的问题。

其实我们只要 emp 表中的 deptno 等于 dept 表中的 deptno 字段的记录，就能够解决数据无效的问题。

• 显示雇员名、雇员工资以及所在部门的名字

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)

• 显示部门号为 10 的部门名，员工名和工资

-- 部门名只在部门表中有,而员工名和工资只有在员工表中有
-- 因此需要将这两表进行组合
mysql> select emp.deptno, emp.ename, emp.sal from emp, dept where emp.deptno = dept.deptno and dept.deptno = 10;
+--------+--------+---------+
| deptno | ename  | sal     |
+--------+--------+---------+
|     10 | CLARK  | 2450.00 |
|     10 | KING   | 5000.00 |
|     10 | 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 * 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)

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

解决多表查询的思路

先明确题目和哪些表有关，然后将这些表组成成一张有效的表，那么多表查询就变成了单表查询了。

👉自连接👈

自连接是指在一个表中通过连接操作自身的过程。通常，自连接在以下场景中应用：

层级结构：当数据表中的记录之间存在层级关系时，可以使用自连接来查询父节点和子节点之间的关系。例如，组织结构表中的每个记录表示一个员工，记录中包含一个指向上级员工的外键，通过自连接可以查询员工和他们的上级。

关联关系：当表中的记录之间存在关联关系时，可以使用自连接来查找相关记录。例如，一个订单表中包含了订单的主订单号和子订单号，通过自连接可以查询主订单和对应的子订单。

数据比较：当需要比较表中的记录时，可以使用自连接来将表中的记录与其他记录进行比较。例如，可以使用自连接来比较表中不同时间段的记录，或者比较相邻行的记录。

别名使用：自连接还可以用于给表起别名，并对表进行多次引用，以便进行更复杂的查询操作。通过别名，可以在同一个查询中将表作为不同的实体进行引用，从而实现更灵活的查询需求。

-- 两张相同的表没经过重命名,无法进行笛卡尔积
mysql> select * from dept, dept;
ERROR 1066 (42000): Not unique table/alias: 'dept'

-- 两张相同的表经过重命名后,可以进行笛卡尔积
mysql> select * from dept as dt1, dept as dt2;
+--------+------------+----------+--------+------------+----------+
| deptno | dname      | loc      | deptno | dname      | loc      |
+--------+------------+----------+--------+------------+----------+
|     10 | ACCOUNTING | NEW YORK |     10 | ACCOUNTING | NEW YORK |
|     20 | RESEARCH   | DALLAS   |     10 | ACCOUNTING | NEW YORK |
|     30 | SALES      | CHICAGO  |     10 | ACCOUNTING | NEW YORK |
|     40 | OPERATIONS | BOSTON   |     10 | ACCOUNTING | NEW YORK |
|     10 | ACCOUNTING | NEW YORK |     20 | RESEARCH   | DALLAS   |
|     20 | RESEARCH   | DALLAS   |     20 | RESEARCH   | DALLAS   |
|     30 | SALES      | CHICAGO  |     20 | RESEARCH   | DALLAS   |
|     40 | OPERATIONS | BOSTON   |     20 | RESEARCH   | DALLAS   |
|     10 | ACCOUNTING | NEW YORK |     30 | SALES      | CHICAGO  |
|     20 | RESEARCH   | DALLAS   |     30 | SALES      | CHICAGO  |
|     30 | SALES      | CHICAGO  |     30 | SALES      | CHICAGO  |
|     40 | OPERATIONS | BOSTON   |     30 | SALES      | CHICAGO  |
|     10 | ACCOUNTING | NEW YORK |     40 | OPERATIONS | BOSTON   |
|     20 | RESEARCH   | DALLAS   |     40 | OPERATIONS | BOSTON   |
|     30 | SALES      | CHICAGO  |     40 | OPERATIONS | BOSTON   |
|     40 | OPERATIONS | BOSTON   |     40 | OPERATIONS | BOSTON   |
+--------+------------+----------+--------+------------+----------+
16 rows in set (0.01 sec)

• 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
-- 使用子查询

mysql> select empno, ename from emp where 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(自连接)
-- 员工领导编号等于领导的编号就能够得到一个有效的表
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)

👉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

单行子查询是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

多行子查询是返回多行记录的子查询。

• 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 (0.01 sec)

-- select distinct job from emp where deptno=10 的返
-- 回结果是10号部门中所包含的岗位
-- in关键字是用来判断job是否在10号部门所包含的岗位集合中,如果在则为真

• 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

-- 30号部门的员工的最高工资
mysql> select max(sal) from emp where deptno=30;
+----------+
| max(sal) |
+----------+
|  2850.00 |
+----------+
1 row in set (0.00 sec)

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 (0.00 sec)

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)

-- select sal from emp where deptno=30 的返回结果是单列多行的数据,这些数据组成了一个集合

• 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）

-- 30号部门的最低工资
mysql> select min(sal) from emp where deptno=30;
+----------+
| min(sal) |
+----------+
|   950.00 |
+----------+
1 row in set (0.00 sec)

mysql> select ename, sal, deptno from emp where sal > (select min(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)

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。

• 查询和 SMITH 的部门和岗位完全相同的所有雇员，不含 SMITH 本人

-- 查看SMITH所在的部门和岗位
mysql> select job, deptno from emp where ename='SMITH';
+-------+--------+
| job   | deptno |
+-------+--------+
| CLERK |     20 |
+-------+--------+
1 row in set (0.00 sec)

mysql> select ename, job, deptno from emp where (job, deptno) = (select job, deptno from emp where ename='SMITH') and ename!='SMITH';
+-------+-------+--------+
| ename | job   | deptno |
+-------+-------+--------+
| ADAMS | CLERK |     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。

-- 使用select语句形成的表也可以被看做一张表,尽管这张表
-- 并不在磁盘中真实存在,同时我们也就有对这张表进行重命名
-- 也可以其他的表进行笛卡尔积
mysql> select * from emp where deptno=20;
+--------+-------+---------+------+---------------------+---------+------+--------+
| 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)

mysql> select * from (select * from emp where deptno=20) as temp 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 |
+--------+-------+-------+------+---------------------+--------+------+--------+
1 row in set (0.00 sec)

结论：MySQL 表的大一统思想：不管我们有多少张表，所有通过 select 查出来的结果，都是一张表。

• 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 deptno, format(avg(sal), 2) myavg  from emp group by deptno;
+--------+----------+
| deptno | myavg    |
+--------+----------+
|     10 | 2,916.67 |
|     20 | 2,175.00 |
|     30 | 1,566.67 |
+--------+----------+
3 rows in set (0.00 sec)

mysql> select ename, emp.deptno, sal, format(myavg,2) from (emp, (select deptno, avg(sal) myavg from emp group by deptno) as temp)
    -> where emp.deptno = temp.deptno and sal > myavg;
+-------+--------+---------+-----------------+
| ename | deptno | sal     | format(myavg,2) |
+-------+--------+---------+-----------------+
| ALLEN |     30 | 1600.00 | 1,566.67        |
| JONES |     20 | 2975.00 | 2,175.00        |
| BLAKE |     30 | 2850.00 | 1,566.67        |
| SCOTT |     20 | 3000.00 | 2,175.00        |
| KING  |     10 | 5000.00 | 2,916.67        |
| FORD  |     20 | 3000.00 | 2,175.00        |
+-------+--------+---------+-----------------+
6 rows in set (0.00 sec)

-- 在上面的表的基础上,再列出这些人的薪资等级
mysql> select * from salgrade, (select ename, emp.deptno, sal, format(myavg,2) from (emp, (select deptno, avg(sal) myavg from emp group by deptno) as temp)
    -> where emp.deptno = temp.deptno and sal > myavg) as ret where ret.sal between losal and hisal;
+-------+-------+-------+-------+--------+---------+-----------------+
| grade | losal | hisal | ename | deptno | sal     | format(myavg,2) |
+-------+-------+-------+-------+--------+---------+-----------------+
|     3 |  1401 |  2000 | ALLEN |     30 | 1600.00 | 1,566.67        |
|     4 |  2001 |  3000 | JONES |     20 | 2975.00 | 2,175.00        |
|     4 |  2001 |  3000 | BLAKE |     30 | 2850.00 | 1,566.67        |
|     4 |  2001 |  3000 | SCOTT |     20 | 3000.00 | 2,175.00        |
|     5 |  3001 |  9999 | KING  |     10 | 5000.00 | 2,916.67        |
|     4 |  2001 |  3000 | FORD  |     20 | 3000.00 | 2,175.00        |
+-------+-------+-------+-------+--------+---------+-----------------+
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

mysql> select deptno, max(sal) mymax from emp group by deptno;
+--------+---------+
| deptno | mymax   |
+--------+---------+
|     10 | 5000.00 |
|     20 | 3000.00 |
|     30 | 2850.00 |
+--------+---------+
3 rows in set (0.00 sec)

mysql> select emp.ename, emp.sal, emp.deptno from emp, (select deptno, max(sal) mymax from emp group by deptno) as temp
    -> where emp.deptno=temp.deptno and emp.sal=temp.mymax;
+-------+---------+--------+
| ename | sal     | deptno |
+-------+---------+--------+
| BLAKE | 2850.00 |     30 |
| SCOTT | 3000.00 |     20 |
| KING  | 5000.00 |     10 |
| FORD  | 3000.00 |     20 |
+-------+---------+--------+
4 rows in set (0.00 sec)

-- 在上表的基础上,找出这些人的工作地点
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 dept.deptno, dname, loc, ename, sal from dept, (select emp.ename, emp.sal, emp.deptno from emp, 
     > (select deptno, max(sal) mymax from emp group by depttno) as temp where emp.deptno=temp.deptno and emp.sal=temp.mymax) as ret 
     > where ret.deptno = dept.deptno;
+--------+------------+----------+-------+---------+
| deptno | dname      | loc      | ename | sal     |
+--------+------------+----------+-------+---------+
|     30 | SALES      | CHICAGO  | BLAKE | 2850.00 |
|     20 | RESEARCH   | DALLAS   | SCOTT | 3000.00 |
|     10 | ACCOUNTING | NEW YORK | KING  | 5000.00 |
|     20 | RESEARCH   | DALLAS   | FORD  | 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 dname, dept.deptno, loc, num from dept, (select deptno, count(*) num from emp group by deptno) temp
    -> where dept.deptno = temp.deptno;
+------------+--------+----------+-----+
| dname      | deptno | loc      | num |
+------------+--------+----------+-----+
| ACCOUNTING |     10 | NEW YORK |   3 |
| RESEARCH   |     20 | DALLAS   |   5 |
| SALES      |     30 | CHICAGO  |   6 |
+------------+--------+----------+-----+
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

-- 有些人既是MANAGER,工资也是超过2500
-- 此时就会存在重复的数据,而union会将重
-- 复的数据去重掉,只保留一条数据
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。

• 将工资大于 2500 或职位是 MANAGER 的人找出来

-- 可以看到JONES和BLAKE的数据是重复的
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)

union 和 union all 的本质就是将多个结果合并成一个结果，也就是将多张表合并成一张表。不过需要注意的是，多张表的列属性需要是一致的。

👉总结👈

本篇博客主要讲解了多表查询、自连接和子查询等等。以上就是本篇博客的全部内容了，如果大家觉得有收获的话，可以点个三连支持一下！谢谢大家啦！💖💝❣️