题目：给如下两个表，写一个查询语句，求出在每一个工资发放日，每个部门的平均工资与公司的平均工资的比较结果 （高 / 低 / 相同）。
1. 不同月份、不同部门的平均工资（group by 两个字段）
select date_format(pay_date,'%Y-%m') as pay_month,department_id,avg(amount) as e_m_d_avg
from salary s left join employee e
on s.employee_id=e.employee_id
group by department_id,pay_month
2. 不同月份、公司的平均工资
select date_format(pay_date,'%Y-%m') as pay_m,avg(amount) as company_avg
from salary
group by pay_m
3.拼接、case when .条件. then .A. when .条件. then .B. else .C. end as 字段名
select pay_month,department_id,
case when e_m_d_avg > company_avg then 'higher'
when e_m_d_avg < company_avg then 'lower' else 'same' end as comparison
from(
select date_format(pay_date,'%Y-%m') as pay_month,department_id,avg(amount) as e_m_d_avg
from salary s left join employee e
on s.employee_id=e.employee_id
group by department_id,pay_month
) a,
(
select date_format(pay_date,'%Y-%m') as pay_m,avg(amount) as company_avg
from salary
group by pay_m
) b
where pay_month=pay_m
order by department_id,pay_month;
