MySQL-进阶7-子查询 - select后/where后/from后/ []where后/having后] / exists后面 的相关子查询

/*SQL-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 或者 外查询
   分类1:按子查询出现的位置————

    select 后面:仅仅支持标量子查询(仅能支持一行一列,下面有栗子)

    from 后面:支持表 子查询
        将子查询的结果充当成一张表,要求必须起别名,不然会引起冲突!

    where 或 having 后面: [重点]
        标量子查询   √
        列子查询     √    [还一般搭配着多行操作符使用,in/ any/some/ all ]
        [  in(not in):等于列表中的任意一个
                  弃用 ####  any/some : (和子查询返回的某一个值比较)
                   弃用 #####  all : 和子查询返回的所有值比较
         ]
        行子查询
    {共同特点:
        ①子查询放在小括号里
        ②子查询放在条件的右侧
        ③标量子查询,一般搭配着单行操作符使用
    }

    exists后面(相关子查询)
        exists后面的子查询 #结果返回1或者0;先执行主查询,再执行子查询

    分类2:按结果集的行列数不同:(用的很少)
    标量子查询(结果集)
    .......

*/

#联系: 多表查询 ,查询部门名为SAL 或IT的员工的所有信息
SELECT e.*,d.`department_name`
FROM employees e,departments d
WHERE e.`department_id`=d.`department_id` AND d.department_name IN ('SAL','IT');

#where 或 having 后面: [重点]
   #1.单行子查询(标量子查询)
    #案例1:谁的工资比ABel的高
SELECT e.*
FROM employees e
WHERE e.`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, 显示 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: 返回公司工资最低的员工的last_name ,job_id 和 salary
SELECT *
FROM employees
WHERE salary =(
    SELECT MIN(salary)
    FROM employees
);

#二: select 后面的子查询

    #案例:查询每个部门的员工个数
SELECT d.*,(
    SELECT COUNT(*)
    FROM employees e
    WHERE d.department_id=e.department_id
) 个数
FROM departments d;
    ##也可以用连接查询???? ：employees 右外连接 departments
SELECT d.*,COUNT(*)
FROM employees e
RIGHT OUTER JOIN departments d
ON e.department_id=d.department_id
GROUP BY department_id;
    #查询员工号=102的部门号
SELECT    (
    SELECT    d.department_name
    FROM departments d,employees e
    WHERE d.department_id = e.department_id
    AND e.employee_id = 102
) 部门名;

#三: from后面
    #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;
    #2.查询工资等级
SELECT * FROM job_grades;
    #3.进行组合 , 连接1的两个结果集和job_grades表,筛选条件平均工资 between low and high
SELECT ag_dep.* ,g.grade_level
FROM (
    SELECT AVG(salary) ag,department_id         #查询后生成的表
    FROM employees
    GROUP BY department_id
) ag_dep
INNER JOIN job_grades g
ON ag_dep.ag BETWEEN g.lowest_sal AND g.highest_sal;

#四: exists后面的(相关子查询)  #结果返回1或者0;先执行主查询,再执行子查询

    #案例1:查询有员工名的部门名
SELECT department_name
FROM departments d
WHERE  EXISTS(
    SELECT *
    FROM employees e
    WHERE    d.department_id = e.`department_id`
);

#----------------栗子练习--------------
    #一: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本部门平均工资
SELECT  AVG(e.`salary`) ag,department_id
FROM employees e
GROUP BY e.`department_id`;
  #(2) 与1进行连接
SELECT e.`department_id`,e.`employee_id`,e.`last_name`,e.`salary`,ag 平均工资
FROM employees e
INNER JOIN(
    SELECT  AVG(e.`salary`) ag,department_id
FROM employees e
GROUP BY e.`department_id`
)ag_dep
ON e.`department_id`=ag_dep.department_id  ##内连接的连接条件!
WHERE salary > ag_dep.ag;        ###加上筛选条件