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子查询

含义：

分类：

• 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）

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 salaryFROM employeesWHERE last_name = 'Abel'#②查询员工的信息，满足 salary>①结果SELECT *FROM employeesWHERE 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_idSELECT job_idFROM employeesWHERE employee_id = 141#②查询143号员工的salarySELECT salaryFROM employeesWHERE 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,salaryFROM employeesWHERE 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=①SELECT last_name,job_id,salaryFROM employeesWHERE 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 employeesWHERE department_id = 50#②查询每个部门的最低工资SELECT MIN(salary),department_idFROM employeesGROUP BY department_id#③ 在②基础上筛选，满足min(salary)>①SELECT MIN(salary),department_idFROM employeesGROUP BY department_idHAVING MIN(salary)>(	SELECT  MIN(salary)	FROM employees	WHERE department_id = 50);#非法使用标量子查询SELECT MIN(salary),department_idFROM employeesGROUP BY department_idHAVING 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_idFROM departmentsWHERE 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_nameFROM employeesWHERE department_id IN(	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 salaryFROM employeesWHERE 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,salaryFROM employeesWHERE salary
   
    'IT_PROG';#或SELECT last_name,employee_id,job_id,salaryFROM employeesWHERE 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salarySELECT last_name,employee_id,job_id,salaryFROM employeesWHERE salary
    
     'IT_PROG';#或SELECT last_name,employee_id,job_id,salaryFROM employeesWHERE 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 employeesWHERE (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 employeesWHERE employee_id=(	SELECT MIN(employee_id)	FROM employees)AND salary=(	SELECT MAX(salary)	FROM employees);
    
   

在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,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;

在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_idFROM employeesGROUP 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_id	FROM employees	GROUP BY department_id) ag_depINNER JOIN job_grades gON ag_dep.ag BETWEEN lowest_sal AND highest_sal;

exists后面（相关子查询）

语法：

SELECT EXISTS(SELECT employee_id FROM employees WHERE salary=300000);#案例1：查询有员工的部门名#inSELECT department_nameFROM departments dWHERE d.`department_id` IN(	SELECT department_id	FROM employees)#existsSELECT department_nameFROM departments dWHERE 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#inSELECT bo.*FROM boys boWHERE bo.id NOT IN(	SELECT boyfriend_id	FROM beauty)#existsSELECT bo.*FROM boys boWHERE NOT EXISTS(	SELECT boyfriend_id	FROM beauty b	WHERE bo.`id`=b.`boyfriend_id`);

#1.	查询和Zlotkey相同部门的员工姓名和工资#①查询Zlotkey的部门SELECT department_idFROM employeesWHERE last_name = 'Zlotkey'#②查询部门号=①的姓名和工资SELECT last_name,salaryFROM employeesWHERE 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查询平均工资SELECT AVG(salary)FROM employees#②查询工资>①的员工号，姓名和工资。SELECT last_name,employee_id,salaryFROM employeesWHERE 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_idFROM employeesGROUP BY department_id#②连接①结果集和employees表，进行筛选SELECT employee_id,last_name,salary,e.department_idFROM employees eINNER JOIN (	SELECT AVG(salary) ag,department_id	FROM employees	GROUP BY department_id) ag_depON e.department_id = ag_dep.department_idWHERE salary>ag_dep.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SELECT  DISTINCT department_idFROM employeesWHERE last_name LIKE '%u%'#②查询部门号=①中的任意一个的员工号和姓名SELECT last_name,employee_idFROM employeesWHERE 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SELECT DISTINCT department_idFROM departments WHERE location_id  = 1700#②查询部门号=①中的任意一个的员工号SELECT employee_idFROM employeesWHERE department_id =ANY(	SELECT DISTINCT department_id	FROM departments 	WHERE location_id  = 1700);#6.查询管理者是King的员工姓名和工资#①查询姓名为king的员工编号SELECT employee_idFROM employeesWHERE last_name  = 'K_ing'#②查询哪个员工的manager_id = ①SELECT last_name,salaryFROM employeesWHERE 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 employeesWHERE salary=(	SELECT MAX(salary)	FROM employees);

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