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）★
操作符含义
IN/NOT IN 等于列表中的任意一个
ANY|SOME 和子查询返回的某一个值比较
ALL 和子查询返回的所有值比较

#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
FROM departments d
INNER JOIN employees e
ON d.department_id=e.department_id
WHERE e.employee_id=102
) 部门名;


---错---
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`
);