解题
select t1.pay_month, t1.department_id, case when t1.am > t2.am then 'higher' when t1.am < t2.am then 'lower' else 'same' end comparisonfrom( select DATE_FORMAT(pay_date,'%Y-%m') pay_month, department_id , avg(amount) am from salary s, employee e where s.employee_id = e.employee_id GROUP BY pay_month, department_id) t1,( select DATE_FORMAT(pay_date,'%Y-%m') pay_month, avg(amount) am from salary GROUP BY pay_month) t2where t1.pay_month = t2.pay_month
`单行代码`