mysql学习笔记(七):子查询

含义：
出现再其他语句中的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和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（结果集一般未多行多列）

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
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
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 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
*/

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 );