含义：
一条查询语句中又嵌套了另一条完整的select语句，其中被嵌套的select语句，称为子查询或内查询
在外面的查询语句，称为主查询或外查询。

分类：

特点：
①子查询放在小括号内
②子查询一般放在条件的右侧
③标量子查询，一般搭配着单行操作符使用 > < >= <= = <> 列子查询，一般搭配着多行操作符使用 in、any/some、all
④子查询的执行优先于主查询执行，主查询的条件用到了子查询的结果

8.1 多行操作符

①in：在范围内的值，只要有就true
②any/some：与子查询返回的任何值比较为true 则返回true
③all：与子查询返回的所有值比较为true 则返回true

8.2 标量子查询

• 结果集只有一行一列 ```sql

  案例1：谁的工资比 Abel 高?

①查询Abel的工资

SELECT salary FROM employees WHERE last_name = ‘Abel’;

②查询员工的信息，满足 salary>①结果

SELECT * FROM employees WHERE salary>(

SELECT salary
FROM employees
WHERE last_name = 'Abel'

);

案例2：返回job_id与141号员工相同，salary比143号员工多的员工 姓名，job_id 和工资

①查询141号员工的job_id

SELECT job_id FROM employees WHERE employee_id = 141;

②查询143号员工的salary

SELECT salary FROM employees WHERE employee_id = 143;

③查询员工的姓名，job_id 和工资，要求job_id=①并且salary>②

SELECT last_name,job_id,salary FROM employees WHERE job_id = ( SELECT job_id FROM employees WHERE employee_id = 141 ) AND salary>( SELECT salary FROM employees WHERE employee_id = 143

);

案例3：返回公司工资最少的员工的last_name,job_id和salary

①查询公司的最低工资

SELECT MIN(salary) FROM employees;

②查询last_name,job_id和salary，要求salary=①

SELECT last_name,job_id,salary FROM employees WHERE salary=( SELECT MIN(salary) FROM employees );

案例4：查询最低工资大于50号部门最低工资的部门id和其最低工资

①查询50号部门的最低工资

SELECT MIN(salary) FROM employees WHERE department_id = 50;

②查询每个部门的最低工资

SELECT MIN(salary),department_id FROM employees GROUP BY department_id;

③ 在②基础上筛选，满足min(salary)>①

SELECT MIN(salary),department_id FROM employees GROUP BY department_id HAVING MIN(salary)>( SELECT MIN(salary) FROM employees WHERE department_id = 50 );

非法使用标量子查询

SELECT MIN(salary),department_id FROM employees GROUP BY department_id HAVING MIN(salary)>( SELECT salary FROM employees WHERE department_id = 250 );

select后面

/ 仅仅支持标量子查询 /

案例：查询每个部门的员工个数

SELECT d.,( SELECT COUNT() FROM employees e WHERE e.department_id = d.department_id ) 个数 FROM departments d;

案例2：查询员工号=102的部门名

SELECT ( SELECT department_name,e.department_id FROM departments d INNER JOIN employees e ON d.department_id=e.department_id WHERE e.employee_id=102

) 部门名;

<a name="OOnPM"></a>
## 8.3 列子查询

- 结果集只有一列多行
```sql
#案例1：返回location_id是1400或1700的部门中的所有员工姓名

#①查询location_id是1400或1700的部门编号
SELECT DISTINCT department_id
FROM departments
WHERE location_id IN(1400,1700);

#②查询员工姓名，要求部门号是①列表中的某一个

SELECT last_name
FROM employees
WHERE department_id  <>ALL(
    SELECT DISTINCT department_id
    FROM departments
    WHERE location_id IN(1400,1700)
);


#案例2：返回其它工种中比job_id为‘IT_PROG’工种任一工资低的员工的员工号、姓名、job_id 以及salary

#①查询job_id为‘IT_PROG’部门任一工资

SELECT DISTINCT salary FROM employees
WHERE job_id = 'IT_PROG';

#②查询员工号、姓名、job_id 以及salary，salary<(①)的任意一个
SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<ANY(
    SELECT DISTINCT salary
    FROM employees
    WHERE job_id = 'IT_PROG'

) AND job_id<>'IT_PROG';

#或
SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<(
    SELECT MAX(salary)
    FROM employees
    WHERE job_id = 'IT_PROG'

) AND job_id<>'IT_PROG';


#案例3：返回其它部门中比job_id为‘IT_PROG’部门所有工资都低的员工   的员工号、姓名、job_id 以及salary

SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<ALL(
    SELECT DISTINCT salary
    FROM employees
    WHERE job_id = 'IT_PROG'

) AND job_id<>'IT_PROG';

#或

SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<(
    SELECT MIN( salary)
    FROM employees
    WHERE job_id = 'IT_PROG'

) AND job_id<>'IT_PROG';

8.4 行子查询

• 结果集只有一行多列 ```sql

  案例：查询员工编号最小并且工资最高的员工信息

SELECT * FROM employees WHERE (employee_id,salary)=( SELECT MIN(employee_id),MAX(salary) FROM employees );

①查询最小的员工编号

SELECT MIN(employee_id) FROM employees;

②查询最高工资

SELECT MAX(salary) FROM employees;

③查询员工信息

SELECT * FROM employees WHERE employee_id=( SELECT MIN(employee_id) FROM employees )AND salary=( SELECT MAX(salary) FROM employees );

<a name="sUJ1x"></a>
## 8.5 表子查询

- 结果集一般为多行多列
```sql
#from后面
/*
将子查询结果充当一张表，要求必须起别名
*/

#案例：查询每个部门的平均工资的工资等级
#①查询每个部门的平均工资
SELECT AVG(salary),department_id
FROM employees GROUP BY department_id;

SELECT * FROM job_grades;

#②连接①的结果集和job_grades表，筛选条件平均工资 between lowest_sal and highest_sal

SELECT  ag_dep.*,g.`grade_level`
FROM (
    SELECT AVG(salary) ag,department_id
    FROM employees
    GROUP BY department_id
) ag_dep
INNER JOIN job_grades g
ON ag_dep.ag BETWEEN lowest_sal AND highest_sal;

8.6 exists后面（相关子查询）

• 语法： exists(完整的查询语句)
• 结果： 1或0 ```sql SELECT EXISTS(SELECT employee_id FROM employees WHERE salary=300000);

案例1：查询有员工的部门名

in

SELECT department_name FROM departments d WHERE d.department_id IN( SELECT department_id FROM employees );

exists

SELECT department_name FROM departments d WHERE EXISTS( SELECT * FROM employees e WHERE d.department_id=e.department_id );

案例2：查询没有女朋友的男神信息

in

SELECT bo.* FROM boys bo WHERE bo.id NOT IN( SELECT boyfriend_id FROM beauty );

exists

SELECT bo.* FROM boys bo WHERE NOT EXISTS( SELECT boyfriend_id FROM beauty b WHERE bo.id=b.boyfriend_id );

```