#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-----即子查询结果必须是一行一列
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
#行子查询
SELECT *
FROM employees
WHERE (employee_id,salary)=(
SELECT MIN(employee_id),MAX(salary)
FROM employees
);
#普通解法
#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-----即子查询结果只能是一行一列
*/
#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
*/
#主要用于子语句的结果是否存在
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`
);
#---------子查询相关案例-----------#
#1.查询和Zlotkey相同部门的员工姓名和工资
#(1)查询和Zlotkey相同部门id
SELECT department_id
FROM employees
WHERE last_name = 'Zlotkey';
#(2)查询员工姓名和工资
SELECT `last_name`,`salary`
FROM employees
WHERE department_id=(
SELECT department_id
FROM employees
WHERE last_name = 'Zlotkey'
);
#2.查询工资比公司平均工资高的员工的员工号，姓名和工资。
#(1)查询公司平均工资
SELECT AVG(salary)
FROM employees;
#(2)查询工资比公司平均工资高的员工的员工号，姓名和工资
SELECT `employee_id`,`last_name`,`salary`
FROM employees
WHERE `salary`>(
SELECT AVG(salary)
FROM employees
);
#3.查询各部门中工资比本部门平均工资高的员工的员工号, 姓名和工资
#(1)查询各部门的平均工资
SELECT AVG(salary),`department_id`
FROM `employees`
GROUP BY `department_id`;
#(2)查询各部门中工资比本部门平均工资高的员工的员工号, 姓名和工资
SELECT e.`employee_id`,e.`last_name`,e.`salary`
FROM `employees` e
JOIN (
SELECT AVG(salary) ag,`department_id`
FROM `employees`
GROUP BY `department_id`
) n
ON e.`department_id`=n.`department_id`
WHERE e.`salary`>n.ag;
#4.查询和姓名中包含字母u的员工在相同部门的员工的员工号和姓名
#(1)查询姓名中包含字母u的员工的部门id
SELECT DISTINCT `department_id`
FROM `employees`
WHERE last_name LIKE '%u%';
#(2)查询和(1)相同部门的员工的员工号和姓名
SELECT `employee_id`,`last_name`
FROM `employees`
WHERE `department_id` IN (
SELECT DISTINCT `department_id`
FROM `employees`
WHERE last_name LIKE '%u%'
);
#5. 查询在部门的location_id为1700的部门工作的员工的员工号
#(1)查询location_id为1700的部门id
SELECT `department_id`
FROM `departments`
WHERE `location_id`=1700;
#(2)查询在部门的location_id为1700的部门工作的员工的员工号
SELECT `employee_id`
FROM `employees`
WHERE `department_id` IN (
SELECT `department_id`
FROM `departments`
WHERE `location_id`=1700
);
#6.查询管理者是K_ing的员工姓名和工资
#(1)查询K_ing的id
SELECT `employee_id`
FROM `employees`
WHERE `last_name`='K_ing';
#(2)查询管理者是K_ing的员工姓名和工资
SELECT m.`last_name`,m.`salary`
FROM `employees` m
WHERE m.`manager_id` IN (
SELECT `employee_id`
FROM `employees`
WHERE `last_name`='K_ing'
);
#7.查询工资最高的员工的姓名，要求first_name和last_name显示为一列，列名为 姓.名
#①查询最高工资
SELECT MAX(salary)
FROM employees
#②查询工资=①的姓.名
SELECT CONCAT(first_name,last_name) "姓.名"
FROM employees
WHERE salary=(
SELECT MAX(salary)
FROM employees
);
#------子查询总结------#
一、含义
嵌套在其他语句内部的SELECT语句称为子查询或内查询，
外面的语句可以是INSERT、UPDATE、DELETE、SELECT等，一般SELECT作为外面语句较多
外面如果为SELECT语句，则此语句称为外查询或主查询
二、分类
1、按出现位置
SELECT后面：
仅仅支持标量子查询
FROM后面：
表子查询
WHERE或HAVING后面：
标量子查询
列子查询
行子查询
EXISTS后面：
标量子查询
列子查询
行子查询
表子查询
2、按结果集的行列
标量子查询（单行子查询）：结果集为一行一列
列子查询（多行子查询）：结果集为多行一列
行子查询：结果集为多行多列
表子查询：结果集为多行多列
三、示例
WHERE或HAVING后面
1、标量子查询
案例：查询最低工资的员工姓名和工资
①最低工资
SELECT MIN(salary) FROM employees
②查询员工的姓名和工资，要求工资=①
SELECT last_name,salary
FROM employees
WHERE salary=(
SELECT MIN(salary) FROM employees
);
2、列子查询
案例：查询所有是领导的员工姓名
①查询所有员工的 manager_id
SELECT manager_id
FROM employees
②查询姓名，employee_id属于①列表的一个
SELECT last_name
FROM employees
WHERE employee_id IN(
SELECT manager_id
FROM employees
);