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  • 进阶5:分组查询

    #进阶5:分组查询

    /*
    语法:

    select 查询列表
    from 表
    【where 筛选条件】
    group by 分组的字段
    【order by 排序的字段】;

    特点:
    1、和分组函数一同查询的字段必须是group by后出现的字段
    2、筛选分为两类:分组前筛选和分组后筛选
    针对的表 位置 连接的关键字
    分组前筛选 原始表 group by前 where

    分组后筛选 group by后的结果集 group by后 having

    问题1:分组函数做筛选能不能放在where后面
    答:不能

    问题2:where——group by——having

    一般来讲,能用分组前筛选的,尽量使用分组前筛选,提高效率

    3、分组可以按单个字段也可以按多个字段
    4、可以搭配着排序使用


    */

    #引入:查询每个部门的员工个数

    SELECT COUNT(*) FROM employees WHERE department_id=90;
    #1.简单的分组

    #案例1:查询每个工种的员工平均工资
    SELECT AVG(salary),job_id
    FROM employees
    GROUP BY job_id;

    #案例2:查询每个位置的部门个数

    SELECT COUNT(*),location_id
    FROM departments
    GROUP BY location_id;


    #2、可以实现分组前的筛选

    #案例1:查询邮箱中包含a字符的 每个部门的最高工资

    SELECT MAX(salary),department_id
    FROM employees
    WHERE email LIKE '%a%'
    GROUP BY department_id;


    #案例2:查询有奖金的每个领导手下员工的平均工资

    SELECT AVG(salary),manager_id
    FROM employees
    WHERE commission_pct IS NOT NULL
    GROUP BY manager_id;

    #3、分组后筛选

    #案例:查询哪个部门的员工个数>5

    #①查询每个部门的员工个数
    SELECT COUNT(*),department_id
    FROM employees
    GROUP BY department_id;

    #② 筛选刚才①结果

    SELECT COUNT(*),department_id
    FROM employees

    GROUP BY department_id

    HAVING COUNT(*)>5;


    #案例2:每个工种有奖金的员工的最高工资>12000的工种编号和最高工资

    SELECT job_id,MAX(salary)
    FROM employees
    WHERE commission_pct IS NOT NULL
    GROUP BY job_id
    HAVING MAX(salary)>12000;


    #案例3:领导编号>102的每个领导手下的最低工资大于5000的领导编号和最低工资

    manager_id>102

    SELECT manager_id,MIN(salary)
    FROM employees
    GROUP BY manager_id
    HAVING MIN(salary)>5000;


    #4.添加排序

    #案例:每个工种有奖金的员工的最高工资>6000的工种编号和最高工资,按最高工资升序

    SELECT job_id,MAX(salary) m
    FROM employees
    WHERE commission_pct IS NOT NULL
    GROUP BY job_id
    HAVING m>6000
    ORDER BY m ;


    #5.按多个字段分组

    #案例:查询每个工种每个部门的最低工资,并按最低工资降序

    SELECT MIN(salary),job_id,department_id
    FROM employees
    GROUP BY department_id,job_id
    ORDER BY MIN(salary) DESC;

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  • 原文地址:https://www.cnblogs.com/Diyo/p/11347091.html
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