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你想知道的 MySQL---三种子查询都在这里！_mysql中from后面可以跟子查询吗

作者：Li_阴宅 | 2024-07-23 20:43:44

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mysql中from后面可以跟子查询吗

一、知识点介绍

什么是子查询？？

很简单，其实就是select语句中嵌套select语句，被嵌套的语句称为子查询

二、分类

子查询都可以出现在哪里呢？

其实子查询在select ，from，where 语句后都可以出现，结构如下：

select

....(select)

from

....(select)

where

....(select)

三、实例展示

1.where子句中的子查询

案例：利用下表找出比最低工资高的员工姓名和工资？


mysql> select * from emp;
+-------+--------+-----------+------+------------+---------+---------+--------+
| EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO |
+-------+--------+-----------+------+------------+---------+---------+--------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |
|  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |
+-------+--------+-----------+------+------------+---------+---------+--------+
14 rows in set (0.01 sec)

实现思路：

第一步：查询最低工资是多少


mysql> select min(sal) from emp;
+----------+
| min(sal) |
+----------+
|   800.00 |
+----------+
1 row in set (0.00 sec)

第二步：找出工资>800的


mysql> select ename as '姓名',sal as '工资' from emp where sal >800;
+--------+---------+
| 姓名   | 工资    |
+--------+---------+
| ALLEN  | 1600.00 |
| WARD   | 1250.00 |
| JONES  | 2975.00 |
| MARTIN | 1250.00 |
| BLAKE  | 2850.00 |
| CLARK  | 2450.00 |
| SCOTT  | 3000.00 |
| KING   | 5000.00 |
| TURNER | 1500.00 |
| ADAMS  | 1100.00 |
| JAMES  |  950.00 |
| FORD   | 3000.00 |
| MILLER | 1300.00 |
+--------+---------+
13 rows in set (0.00 sec)

第三步：合并


mysql> select ename as '姓名',sal as '工资' from emp where sal >(select min(sal) from emp );
+--------+---------+
| 姓名   | 工资    |
+--------+---------+
| ALLEN  | 1600.00 |
| WARD   | 1250.00 |
| JONES  | 2975.00 |
| MARTIN | 1250.00 |
| BLAKE  | 2850.00 |
| CLARK  | 2450.00 |
| SCOTT  | 3000.00 |
| KING   | 5000.00 |
| TURNER | 1500.00 |
| ADAMS  | 1100.00 |
| JAMES  |  950.00 |
| FORD   | 3000.00 |
| MILLER | 1300.00 |
+--------+---------+
13 rows in set (0.00 sec)

2、from 子句中的子查询

注意：from 后面的子查询，可以将子查询的查询结果当作一张临时表。（技巧）

案例：利用下表找出每个岗位的平均工资的薪资等级。（分别为职工表、薪资登记表）


mysql> select * from emp;
+-------+--------+-----------+------+------------+---------+---------+--------+
| EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO |
+-------+--------+-----------+------+------------+---------+---------+--------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |
|  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |
+-------+--------+-----------+------+------------+---------+---------+--------+
14 rows in set (0.01 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 (0.00 sec)

实现思路：

第一步：找出每个岗位的平均工资（按照岗位分组求平均值）


mysql> select job,avg(sal) avgsal from emp group by job;
+-----------+-------------+
| job       | avgsal      |
+-----------+-------------+
| CLERK     | 1037.500000 |
| SALESMAN  | 1400.000000 |
| MANAGER   | 2758.333333 |
| ANALYST   | 3000.000000 |
| PRESIDENT | 5000.000000 |
+-----------+-------------+
5 rows in set (0.00 sec)

第二步：克服心理障碍，把以上的查询结果当作一张真实存在的表，并可以命名为 ‘ t ’

t表和salgrade表进行表连接，条件：t 表avg(sal) between losal and hisal

第三步：合并

（注意：若是在 from 后面的子查询中有聚集函数，则必须要给其取别名，不然会报错）


mysql> select t.*,s.grade
    -> from (select job,avg(sal) avgsal from emp group by job) t
    -> join salgrade s
    -> on t.avgsal between s.losal and s.hisal;
+-----------+-------------+-------+
| job       | avgsal      | grade |
+-----------+-------------+-------+
| CLERK     | 1037.500000 |     1 |
| SALESMAN  | 1400.000000 |     2 |
| MANAGER   | 2758.333333 |     4 |
| ANALYST   | 3000.000000 |     4 |
| PRESIDENT | 5000.000000 |     5 |
+-----------+-------------+-------+
5 rows in set (0.00 sec)

3、select 后面出现的子查询

案例：利用找出每个员工的部门名称，要求显示员工名，部门名？

第一步：先查员工的员工名，部门号


mysql> select e.ename,e.deptno from emp e;
+--------+--------+
| ename  | deptno |
+--------+--------+
| SMITH  |     20 |
| ALLEN  |     30 |
| WARD   |     30 |
| JONES  |     20 |
| MARTIN |     30 |
| BLAKE  |     30 |
| CLARK  |     10 |
| SCOTT  |     20 |
| KING   |     10 |
| TURNER |     30 |
| ADAMS  |     20 |
| JAMES  |     30 |
| FORD   |     20 |
| MILLER |     10 |
+--------+--------+
14 rows in set (0.00 sec)

第二步：在查每个员工的部门名称


mysql> select e.ename,e.deptno,(select d.dname from dept d where e.deptno=d.deptno) as dname from emp e;
+--------+--------+------------+
| ename  | deptno | dname      |
+--------+--------+------------+
| SMITH  |     20 | RESEARCH   |
| ALLEN  |     30 | SALES      |
| WARD   |     30 | SALES      |
| JONES  |     20 | RESEARCH   |
| MARTIN |     30 | SALES      |
| BLAKE  |     30 | SALES      |
| CLARK  |     10 | ACCOUNTING |
| SCOTT  |     20 | RESEARCH   |
| KING   |     10 | ACCOUNTING |
| TURNER |     30 | SALES      |
| ADAMS  |     20 | RESEARCH   |
| JAMES  |     30 | SALES      |
| FORD   |     20 | RESEARCH   |
| MILLER |     10 | ACCOUNTING |
+--------+--------+------------+
14 rows in set (0.00 sec)

注意：对于select后面的子查询来说，一次只能返回一条结果，多余一条记录，就报错了！

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你想知道的 MySQL---三种子查询 都在这里！_mysql中from后面可以跟子查询吗

一、知识点介绍

二、分类

三、实例展示

1.where子句中的子查询

案例：利用下表找出比最低工资高的员工姓名和工资？

实现思路：

第一步：查询最低工资是多少

第二步：找出工资>800的

第三步：合并

2、from 子句中的子查询

案例： 利用下表找出每个岗位的平均工资的薪资等级。（分别为职工表、薪资登记表）

实现思路：

3、select 后面出现的子查询

你想知道的 MySQL---三种子查询都在这里！_mysql中from后面可以跟子查询吗

案例：利用下表找出每个岗位的平均工资的薪资等级。（分别为职工表、薪资登记表）