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、all④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 salaryFROM employeesWHERE 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) agFROM `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) agFROM `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`
);