八：子查询（where、having）

#进阶七：子查询

/*

说明：当一个查询语句中又嵌套了另一个完整的select语句，则被嵌套的select语句称为子查询或内查询

外面的select语句称为主查询或外查询。

分类：

按子查询出现的位置进行分类：

1、select后面

    要求：子查询的结果为单行单列（标量子查询）

2、from后面

    要求：子查询的结果可以为多行多列

3、where或having后面 ★

    要求：子查询的结果必须为单列

        单行子查询

        多行子查询

4、exists后面

    要求：子查询结果必须为单列（相关子查询）    

特点：

    1、子查询放在条件中，要求必须放在条件的右侧

    2、子查询一般放在小括号中

    3、子查询的执行优先于主查询

    4、单行子查询对应了 单行操作符：> < >= <= = <>

       多行子查询对应了 多行操作符：any/some  all in            

#一、放在where或having后面

#一）单行子查询

#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salary>①的员工信息

SELECT last_name,salary
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job_id=① and salary>②的信息

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查询最低工资

SELECT MIN(salary)
FROM employees

#②查询salary=①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

#①查询50号部门的最低工资

SELECT MIN(salary)
FROM employees
WHERE department_id = 50

#②查询各部门的最低工资，筛选看哪个部门的最低工资>①

SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
    SELECT MIN(salary)
    FROM employees
    WHERE department_id = 50
);

练习：

#1. 查询和 Zlotkey 相同部门的员工姓名和工资

#①查询Zlotkey的部门编号

SELECT department_id
FROM employees
WHERE last_name = 'Zlotkey'

#②查询department_id = ①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。

#①查询平均工资

SELECT AVG(salary)
FROM employees

#②查询salary>①的信息

SELECT employee_id,last_name,salary
FROM employees
WHERE salary>(
    SELECT AVG(salary)
    FROM employees
);         

                                                                       

#二）多行子查询

/*

in:判断某字段是否在指定列表内  

x  in(10,30,50)

any / some:判断某字段的值是否满足其中任意一个

x>any(10,30,50)      等价于      x>min()
x=any(10,30,50)      等价于      x in(10,30,50)

all:判断某字段的值是否满足里面所有的

x >all(10,30,50)        等价于     x >max()

*/

#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

SELECT department_id
FROM departments
WHERE location_id IN(1400,1700)

#②查询department_id = ①的姓名

SELECT last_name 员工姓名
FROM employees
WHERE department_id IN(
    SELECT DISTINCT department_id
    FROM departments
    WHERE location_id IN(1400,1700)
);

#题目：返回其它部门中比job_id为‘IT_PROG’部门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’部门的工资

SELECT DISTINCT salary
FROM employees
WHERE job_id = 'IT_PROG'

#②查询其他部门的工资<任意一个①的结果

SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary<ANY(
    SELECT DISTINCT salary
    FROM employees
    WHERE job_id = 'IT_PROG'
);

等价于

SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary<(
    SELECT MAX(salary)
    FROM employees
    WHERE 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

#①查询job_id为‘IT_PROG’部门的工资

SELECT DISTINCT salary
FROM employees
WHERE job_id = 'IT_PROG'

#②查询其他部门的工资<所有①的结果

SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary<ALL(
    SELECT DISTINCT salary
    FROM employees
    WHERE job_id = 'IT_PROG'
);

等价于

SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary<(
    SELECT MIN(salary)
    FROM employees
    WHERE job_id = 'IT_PROG'
);          

                                        

#二、放在select后面

#案例；查询部门编号是50的员工个数

#方式一;

 SELECT COUNT(*)
  FROM `employees`
  WHERE `department_id`=50;

#方式二：

 SELECT 
 (
     SELECT COUNT(*)
     FROM employees
     WHERE department_id = 50
)  个数;

#三、放在from后面

#案例：查询每个部门的平均工资的工资级别

#①查询每个部门的平均工资

  SELECT AVG(salary),department_id
  FROM employees
  GROUP BY department_id

#②将①和sal_grade两表连接查询

  SELECT dep_ag.department_id,dep_ag.ag,g.grade
  FROM sal_grade g
  JOIN (
     SELECT AVG(salary) ag,department_id
      FROM employees
      GROUP BY department_id
) dep_ag ON dep_ag.ag BETWEEN g.min_salary AND g.max_salary;

#四、放在exists后面

#案例1 ：查询有无名字叫“Abel”的员工信息

  SELECT EXISTS(
      SELECT * 
      FROM employees
      WHERE last_name = 'Abel'
 ) 有无Abel;

#案例2：查询没有女朋友的男神信息

 USE girls;
  SELECT bo.*
  FROM boys bo
  WHERE bo.`id` NOT IN(
      SELECT boyfriend_id
      FROM beauty b
 )