select后面

案例:查询每个部门的员工个数

# 案例:查询每个部门的员工个数
SELECT d.*,(
	SELECT COUNT(*)
	FROM `employees` e
	WHERE e.`department_id`=d.`department_id`
) 个数
FROM `departments` d;

案例:查询员工号=102的部门名

# 案例:查询员工号=102的部门名
select (
	select `department_name`
	from `departments` d
	inner join `employees` e
	on e.`department_id`=d.`department_id`
	where e.`employee_id`=102
) 部门名;

三、from后面

案例:查询每个部门的平均工资的工资等级

先查询每个部门的平均工资

# ① 查询每个部门的平均工资
select avg(`salary`),`department_id`
from `employees` e
group by `department_id`

然后②连接①的结果集和job_grades表，筛选条件平均工资between lowest_sal and highest_sal

#②连接①的结果集和job_grades表，筛选条件平均工资between lowest_sal and highest_sal
select ag_dep.*,g.`grade_level`
from (
	select avg(`salary`) ag,`department_id`
	from `employees` e
	group by `department_id`
)ag_dep
inner join `job_grades` g
on ag_dep.ag between lowest_sal and highest_sal;