语法:
select 分组函数，列（要求出现在group by的后面）
from 表
where ..
group by 分组的列表
order by ...

#案例：查询各个部门的平均工资

SELECT AVG(salary),department_idFROM employeesGROUP BYdepartment_idORDER BYdepartment_id DESC;

#案例:查询每个工种的最高工资

SELECTMAX(salary),job_idFROMemployeesGROUP BYjob_id;

#案例:查询每个部门的位置个数

SELECTCOUNT(location_id),department_nameFROMdepartmentsGROUP BYdepartment_name;

#案例:查询邮箱中包含a字符的，每个部门的平均工资

SELECTAVG(salary),department_idFROMemployeesWHEREemail LIKE('%a%')GROUP BYdepartment_id;

#案例：查询有奖金的每个领导手下员工的最高工资

SELECTMAX(salary),manager_idFROM employeesWHEREcommission_pct IS NOT NULLGROUP BY manager_id;