MySQL——子查询

2023.9.8

        相关学习笔记：

#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（重点）
#案例1：谁的工资比 Abel 高?
SELECT `last_name`
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	
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
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
SELECT `department_id`,MIN(salary)
FROM `employees`
GROUP BY `department_id`
HAVING MIN(salary)>(SELECT MIN(salary) FROM `employees` WHERE `department_id`=50);
 
#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
SELECT `last_name`
FROM `employees`
WHERE `department_id` IN 
(SELECT `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
SELECT `employee_id`,`last_name`,`job_id`,`salary`
FROM `employees` 
WHERE salary < ANY
(SELECT 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';
 
#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`);
 
#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.`department_name`,(
SELECT COUNT(*) FROM `employees` e WHERE e.`department_id` = d.`department_id`
) 员工人数
FROM `departments` d;
#案例2：查询员工号=102的部门名
SELECT `department_name`
FROM `departments` d
INNER JOIN  `employees` e
ON d.`department_id` = e.`department_id`
WHERE `employee_id` = 102;
 
#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
SELECT Ag_dep.*,j.`grade_level`
FROM(
	SELECT `department_id`,AVG(salary) ag
	FROM `employees`
	GROUP BY `department_id`
) Ag_dep
INNER JOIN `job_grades` j
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
#用exists实现
SELECT `department_name`
FROM `departments` d
WHERE EXISTS(
SELECT *
FROM `employees` e
WHERE d.`department_id`=e.`department_id`
);
#用in来实现
SELECT `department_name`
FROM `departments` d
WHERE d.`department_id` IN (
SELECT `department_id`
FROM `employees`);
#案例2：查询没有女朋友的男神信息
#in实现
SELECT bo.*
FROM `boys` bo
WHERE bo.`id`  NOT IN (
SELECT `boyfriend_id`
FROM `beauty`);
#exists实现
SELECT bo.*
FROM `boys` bo
WHERE NOT EXISTS(
SELECT `boyfriend_id`
FROM `beauty` b
WHERE bo.`id`=b.`boyfriend_id`);

        课后习题：

#课后习题
#1.查询和Zlotkey相同部门的员工姓名和工资
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。
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
SELECT `employee_id`,`last_name`,`salary`
FROM `employees` e
INNER JOIN (
	SELECT `department_id`,AVG(salary) ag
	FROM `employees`
	GROUP BY `department_id`
) b
ON e.`department_id`=b.`department_id`
WHERE salary > b.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
SELECT `employee_id`,`last_name`
FROM `employees`
WHERE `department_id` IN(
SELECT `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
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King的员工姓名和工资
SELECT `last_name`,`salary`
FROM `employees` 
WHERE `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
SELECT CONCAT(`last_name`,`first_name`) '姓.名'
FROM `employees`
WHERE salary=(
	SELECT MAX(salary)
	FROM `employees`
);

2023.9.11

        更新一下子查询经典案例习题，这些题拿下了子查询基本就掌握了。

#子查询经典案例
# 1. 查询工资最低的员工信息: last_name, salary
SELECT `last_name`,salary
FROM `employees`
WHERE salary = (
	SELECT MIN(salary)
	FROM `employees`
);
 
# 2. 查询平均工资最低的部门信息
# 本人解法(用到两个select)：
# 先查各部门的平均工资，再按平均工资升序排序，再用limit取第一行结果，此时得到的是个一行两列的表。再将此表和`departments`表内连接，连接条件为`department_id`字段相同，即可。
SELECT d.`department_id`,d.`department_name`,d.`manager_id`,d.`location_id`
FROM `departments` d
INNER JOIN 
(SELECT AVG(salary) ag,d.`department_id`
FROM `departments` d
INNER JOIN `employees` e
ON d.`department_id`=e.`department_id`
GROUP BY `department_id`
ORDER BY ag ASC
LIMIT 1) b
ON d.`department_id`=b.`department_id`;
# 上述办法可以优化一下，自己构造的那个表可以只查询`department_id`字段
SELECT *
FROM `departments`
WHERE `department_id`=(
	SELECT `department_id`
	FROM `employees`
	GROUP BY `department_id`
	ORDER BY AVG(salary)
	LIMIT 1
);
#(尚硅谷老师用的方式一这里就不列出来了)
 
# 3. 查询平均工资最低的部门信息和该部门的平均工资
# ps：和上一题差不多，把该部门的平均工资select出来即可
SELECT d.`department_id`,d.`department_name`,d.`manager_id`,d.`location_id`,b.ag
FROM `departments` d
INNER JOIN 
(SELECT AVG(salary) ag,d.`department_id`
FROM `departments` d
INNER JOIN `employees` e
ON d.`department_id`=e.`department_id`
GROUP BY `department_id`
ORDER BY ag ASC
LIMIT 1) b
ON d.`department_id`=b.`department_id`;
 
# 4. 查询平均工资最高的 job 信息
SELECT * FROM `jobs`
WHERE `job_id` = (
	SELECT `job_id` FROM `employees`
	GROUP BY `job_id`
	ORDER BY AVG(`salary`) DESC
	LIMIT 1
);
 
# 5. 查询平均工资高于公司平均工资的部门有哪些?
SELECT `department_id` FROM `employees`
GROUP BY `department_id` 
HAVING AVG(salary) > (
	SELECT AVG(salary) FROM `employees`
);
 
 
# 6. 查询出公司中所有 manager 的详细信息.
SELECT * FROM `employees`
WHERE `employee_id` IN (
	SELECT `manager_id` FROM `employees`
);
 
# 7. 各个部门中 最高工资中最低的那个部门的 最低工资是多少
SELECT MIN(salary) ,department_id
FROM employees
WHERE department_id=(
	SELECT department_id
	FROM employees
	GROUP BY department_id
	ORDER BY MAX(salary)
	LIMIT 1
);
 
# 8. 查询平均工资最高的部门的 manager 的详细信息: last_name, department_id, email, salary
SELECT `last_name`,e.`department_id`,`email`,`salary`
FROM `employees` e
INNER JOIN `departments` d
ON d.`manager_id`=e.`employee_id`
WHERE d.`department_id` = (
	SELECT `department_id` 
	FROM `employees`
	GROUP BY `department_id`
	ORDER BY AVG(salary) DESC
	LIMIT 1
);