mysql子查询

进阶7：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 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 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

);