MySQL复合查询

文章目录

• 一、前情回顾
• 二、多表查询
• 三、自连接
• 四、子查询
• • 1.单行子查询
  • 2.多行子查询
  • 3.多列子查询
  • 4.在from子句中使用子查询
• 五、合并查询

在前面表的基础上进行操作：

mysql> show tables;
+-----------------+
| Tables_in_scott |
+-----------------+
| dept            |
| emp             |
| salgrade        |
+-----------------+
3 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 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 salgrade;
+-------+-------+-------+
| grade | losal | hisal |
+-------+-------+-------+
|     1 |   700 |  1200 |
|     2 |  1201 |  1400 |
|     3 |  1401 |  2000 |
|     4 |  2001 |  3000 |
|     5 |  3001 |  9999 |
+-------+-------+-------+
5 rows in set (0.01 sec)

一、前情回顾

• 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 * from emp where (sal > 500 || 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, 1 warning (0.00 sec)

或者

mysql> select * from emp where (sal > 500 || 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, 1 warning (0.00 sec)

• 按照部门号升序排序，当部门号相同时按雇员的工资降序排序

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

• 使用年薪进行降序排序

注意：NULL与任意值相加都为NULL

mysql> select ename, sal, comm, sal*12+ifnull(comm,0) 年薪 from emp order by sal*12+ifnull(comm,0) desc;
+--------+---------+---------+----------+
| ename  | sal     | comm    | 年薪     |
+--------+---------+---------+----------+
| KING   | 5000.00 |    NULL | 60000.00 |
| SCOTT  | 3000.00 |    NULL | 36000.00 |
| FORD   | 3000.00 |    NULL | 36000.00 |
| JONES  | 2975.00 |    NULL | 35700.00 |
| BLAKE  | 2850.00 |    NULL | 34200.00 |
| CLARK  | 2450.00 |    NULL | 29400.00 |
| ALLEN  | 1600.00 |  300.00 | 19500.00 |
| TURNER | 1500.00 |    0.00 | 18000.00 |
| MARTIN | 1250.00 | 1400.00 | 16400.00 |
| MILLER | 1300.00 |    NULL | 15600.00 |
| WARD   | 1250.00 |  500.00 | 15500.00 |
| ADAMS  | 1100.00 |    NULL | 13200.00 |
| JAMES  |  950.00 |    NULL | 11400.00 |
| SMITH  |  800.00 |    NULL |  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

两条语句：

mysql> select max(sal) from emp;
+----------+
| max(sal) |
+----------+
|  5000.00 |
+----------+
1 row in set (0.00 sec)

mysql> select * from emp where sal = 5000;
+--------+-------+-----------+------+---------------------+---------+------+--------+
| empno  | ename | job       | mgr  | hiredate            | sal     | comm | deptno |
+--------+-------+-----------+------+---------------------+---------+------+--------+
| 007839 | KING  | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL |     10 |
+--------+-------+-----------+------+---------------------+---------+------+--------+
1 row in set (0.00 sec)

一条语句：执行时，先执行（）内的子查询

mysql> select * from emp where sal = (select max(sal) from emp);
+--------+-------+-----------+------+---------------------+---------+------+--------+
| empno  | ename | job       | mgr  | hiredate            | sal     | comm | deptno |
+--------+-------+-----------+------+---------------------+---------+------+--------+
| 007839 | KING  | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL |     10 |
+--------+-------+-----------+------+---------------------+---------+------+--------+
1 row in set (0.00 sec)

• 显示工资高于平均工资的员工信息

mysql>  select * from emp where sal > (select avg(sal) from emp);
+--------+-------+-----------+------+---------------------+---------+------+--------+
| 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 | 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 |
| 007902 | FORD  | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 |
+--------+-------+-----------+------+---------------------+---------+------+--------+
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, avg(sal) 平均工资, max(sal) 最高工资 from emp group by deptno;
+--------+--------------+--------------+
| deptno | 平均工资     | 最高工资     |
+--------+--------------+--------------+
|     20 |  2175.000000 |      3000.00 |
|     30 |  1566.666667 |      2850.00 |
|     10 |  2916.666667 |      5000.00 |
+--------+--------------+--------------+
3 rows in set (0.00 sec)

更美观一些：

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     |
+--------+--------------+--------------+
|     20 | 2,175.00     |      3000.00 |
|     30 | 1,566.67     |      2850.00 |
|     10 | 2,916.67     |      5000.00 |
+--------+--------------+--------------+
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

 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)

为什么格式化了之后不对呢？

mysql> select deptno, format(avg(sal), 2) 平均工资 from emp group by deptno having 平均工资 < 2000;
+--------+--------------+
| deptno | 平均工资     |
+--------+--------------+
|     20 | 2,175.00     |
|     30 | 1,566.67     |
|     10 | 2,916.67     |
+--------+--------------+
3 rows in set, 3 warnings (0.00 sec)

因为 ：由于使用了FORMAT 函数，平均工资被格式化为带有逗号的字符串 （例如2,175.00）。在HAVING子句中，将这个字符串与数字2000比较时，MySQL会尝试将字符串转换为数字 。但是，由于字符串中包含逗号，转换可能会失败，实际上MySQL会截取字符串中直到第一个非数字字符（逗号）之前的数字部分 ，所以"2,175.00"会被转换为2，因此条件"平均工资 < 2000"对于所有行都成立

• 显示每种岗位的雇员总数，平均工资

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

二、多表查询

当数据来自不同的表时，我们需要用到多表查询

• 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表，因此要多表查询

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 (0.01 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)

多表查询的本质 是两个表之间将数据进行穷举组合 即笛卡尔积，即emp表中的第一条信息分别与dept表中的每一条信息进行整合，然后emp表中第二条信息分别与dept表中的每一条信息进行整合...以此类推

注意: 笛卡尔积之后的结果有一些数据是无效的，因此我们需要加上过滤条件

• 显示部门号为10的部门名，员工名和工资

mysql> select emp.deptno, ename, sal, dname from emp, dept where emp.deptno = dept.deptno && emp.deptno = 10;
+--------+--------+---------+------------+
| deptno | ename  | sal     | dname      |
+--------+--------+---------+------------+
|     10 | CLARK  | 2450.00 | ACCOUNTING |
|     10 | KING   | 5000.00 | ACCOUNTING |
|     10 | MILLER | 1300.00 | ACCOUNTING |
+--------+--------+---------+------------+
3 rows in set, 1 warning (0.00 sec)

• 显示各个员工的姓名，工资，及工资级别

需要将emp表与salgrade表结合

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)

三、自连接

自连接是将同一张表做笛卡尔积 ，此时至少给一张表起别名

mysql> select * from dept, dept;
ERROR 1066 (42000): Not unique table/alias: 'dept'
mysql> select * from dept d1, dept;
+--------+------------+----------+--------+------------+----------+
| deptno | dname      | loc      | deptno | dname      | loc      |
+--------+------------+----------+--------+------------+----------+
|     40 | OPERATIONS | BOSTON   |     10 | ACCOUNTING | NEW YORK |
|     30 | SALES      | CHICAGO  |     10 | ACCOUNTING | NEW YORK |
|     20 | RESEARCH   | DALLAS   |     10 | ACCOUNTING | NEW YORK |
|     10 | ACCOUNTING | NEW YORK |     10 | ACCOUNTING | NEW YORK |
|     40 | OPERATIONS | BOSTON   |     20 | RESEARCH   | DALLAS   |
|     30 | SALES      | CHICAGO  |     20 | RESEARCH   | DALLAS   |
|     20 | RESEARCH   | DALLAS   |     20 | RESEARCH   | DALLAS   |
|     10 | ACCOUNTING | NEW YORK |     20 | RESEARCH   | DALLAS   |
|     40 | OPERATIONS | BOSTON   |     30 | SALES      | CHICAGO  |
|     30 | SALES      | CHICAGO  |     30 | SALES      | CHICAGO  |
|     20 | RESEARCH   | DALLAS   |     30 | SALES      | CHICAGO  |
|     10 | ACCOUNTING | NEW YORK |     30 | SALES      | CHICAGO  |
|     40 | OPERATIONS | BOSTON   |     40 | OPERATIONS | BOSTON   |
|     30 | SALES      | CHICAGO  |     40 | OPERATIONS | BOSTON   |
|     20 | RESEARCH   | DALLAS   |     40 | OPERATIONS | BOSTON   |
|     10 | ACCOUNTING | NEW YORK |     40 | OPERATIONS | BOSTON   |
+--------+------------+----------+--------+------------+----------+
16 rows in set (0.00 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 emp.empno=(select mgr from emp where ename='FORD ');
+--------+-------+
| empno  | ename |
+--------+-------+
| 007566 | JONES |
+--------+-------+
1 row in set (0.00 sec)

使用自连接：

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)

四、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

1.单行子查询

• 显示SMITH同一部门的员工

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

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 |
+-------+---------+---------+--------+
| SMITH | CLERK   |  800.00 |     20 |
| JONES | MANAGER | 2975.00 |     20 |
| BLAKE | MANAGER | 2850.00 |     30 |
| 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
  即比部门30的最高工资还高的

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）
  即比部门30的最低工资高的

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)

3.多列子查询

1. 单行子查询是指子查询只返回单列，单行数据；
2. 多行子查询是指返回单列多行数据，都是针对单列而言的;
3. 多列子查询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

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.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。

• 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, format(sal,2) from emp,
    -> (select avg(sal) asal, deptno dt from emp group by deptno) tmp
    -> where emp.sal > tmp.asal and emp.deptno=tmp.dt;
+-------+--------+---------+---------------+
| ename | deptno | sal     | format(sal,2) |
+-------+--------+---------+---------------+
| FORD  |     20 | 3000.00 | 3,000.00      |
| SCOTT |     20 | 3000.00 | 3,000.00      |
| JONES |     20 | 2975.00 | 2,975.00      |
| BLAKE |     30 | 2850.00 | 2,850.00      |
| ALLEN |     30 | 1600.00 | 1,600.00      |
| KING  |     10 | 5000.00 | 5,000.00      |
+-------+--------+---------+---------------+
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、最高工资

mysql> select emp.ename, emp.sal, emp.deptno, ms from emp,
    -> (select max(sal) 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
  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.deptno, dept.dname, dept.loc;
+------------+--------+----------+--------------+
| dname      | deptno | loc      | 部门人数     |
+------------+--------+----------+--------------+
| RESEARCH   |     20 | DALLAS   |            5 |
| SALES      |     30 | CHICAGO  |            6 |
| ACCOUNTING |     10 | NEW YORK |            3 |
+------------+--------+----------+--------------+
3 rows in set (0.00 sec)

使用子查询：

#1.对emp表进行人员统计
mysql> select count(*), deptno from  emp group by deptno ;
+----------+--------+
| count(*) | deptno |
+----------+--------+
|        5 |     20 |
|        6 |     30 |
|        3 |     10 |
+----------+--------+
3 rows in set (0.00 sec)

#2.将上面的表看作临时表
mysql> select dept.deptno, dname, mycnt, loc from dept,
    -> (select count(*) mycnt, deptno from  emp group by deptno) 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)

五、合并查询

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操作符; 将工资大于2500或职位是MANAGER的人找出来
  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

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操作符; 将工资大于25000或职位是MANAGER的人找出来
  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)