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,分组前使用where,分组后使用having
3、分组可以按单个字段也可以按多个字段
4、可以搭配order by排序使用
数据库结构图:
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;
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;