MySQL 高级篇- 数据库 sql99 子查询

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#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、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★#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 salaryFROM employeesWHERE 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>②SELECT last_name,job_id,salary
FROM employees
WHERE job_id = (SELECT job_idFROM employeesWHERE employee_id = 141
) AND salary>(SELECT salaryFROM employeesWHERE 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 employeesWHERE 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  salaryFROM employeesWHERE 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_idFROM departmentsWHERE 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 salaryFROM employeesWHERE 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 employeesWHERE 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salarySELECT 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';#或SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<(SELECT MIN( salary)FROM 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
);#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 eWHERE 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_idFROM departments dINNER JOIN employees eON d.department_id=e.department_idWHERE 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_idSELECT * 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_salSELECT  ag_dep.*,g.`grade_level`
FROM (SELECT AVG(salary) ag,department_idFROM employeesGROUP 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_idFROM employees)#existsSELECT department_name
FROM departments d
WHERE EXISTS(SELECT *FROM employees eWHERE d.`department_id`=e.`department_id`);#案例2：查询没有女朋友的男神信息#inSELECT bo.*
FROM boys bo
WHERE bo.id NOT IN(SELECT boyfriend_idFROM beauty
)#exists
SELECT bo.*
FROM boys bo
WHERE NOT EXISTS(SELECT boyfriend_idFROM beauty bWHERE bo.`id`=b.`boyfriend_id`);

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