mysql的子查询

/*

含义：

出现在其他语句中的select语句，称为子查询或内查询

外部的查询语句，称为主查询或外查询

 

分类：

按子查询出现的位置：

select后面：

仅仅支持标量子查询

 

from后面：

支持表子查询

where或having后面：★

标量子查询（单行） √

列子查询  （多行） √

 

行子查询

 

exists后面（相关子查询）

表子查询

按结果集的行列数不同：

标量子查询（结果集只有一行一列）

列子查询（结果集只有一列多行）

行子查询（结果集有一行多列）

表子查询（结果集一般为多行多列）

*/

 

#一、where或having后面

/*

1、标量子查询（单行子查询）

2、列子查询（多行子查询）

3、行子查询（多列多行）

 

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

*/

#1.标量子查询★

#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

);

 

 

 

#2.列子查询（多行子查询）★

#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';

 

#3、行子查询（结果集一行多列或多行多列）

 

#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

);

 

#二、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;

 

#三、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;

 

#四、exists后面（相关子查询）

/*

语法：

exists(完整的查询语句)

结果：

1或0

*/

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`

 

);