mysql分组查询

 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

#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

SELECT manager_id,MIN(salary)
FROM employees
WHERE manger_id > 102
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;