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
使用关键字 筛选的表 位置
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
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 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;
练习：
1.查询各job_id的员工工资的最大值，最小值，平均值，总和，并按job_id升序
SELECT MAX(salary),MIN(salary),AVG(salary),SUM(salary),job_id FROM employees GROUP BY job_id ORDER BY job_id;
2.查询员工最高工资和最低工资的差距（DIFFERENCE）
SELECT MAX(salary)-MIN(salary) DIFFRENCE FROM employees;
3.查询各个管理者手下员工的最低工资，其中最低工资不能低于6000，没有管理者的员工不计算在内
SELECT MIN(salary),manager_id FROM employees WHERE manager_id IS NOT NULL GROUP BY manager_id HAVING MIN(salary)>=6000;
4.查询所有部门的编号，员工数量和工资平均值,并按平均工资降序
SELECT department_id,COUNT(*),AVG(salary) a FROM employees GROUP BY department_id ORDER BY a DESC;
5.选择具有各个job_id的员工人数
SELECT COUNT(*) 个数,job_id FROM employees GROUP BY job_id;
