MySQL（七）DQL之分组查询

一、语法

select 分组函数，分组后的字段from 表【where 筛选条件】group by 分组的字段【having 分组后的筛选】【order by 排序列表】

二、特点

分组前筛选：where -   原始表 - group by的前面

分组后筛选：having -  分组后的结果 - group by 的后面

三、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;

 

四、分组前筛选

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;

 

五、分组后筛选

案例1：查询哪个部门的员工个数>5

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 manager_id > 102 GROUP BY manager_id HAVING MIN(salary)>5000;

 

六、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;

 

七、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;

 

关注公众号：Java后端生活，干货文章第一时间送达！