# 这题的数据挺有意思，会有人上班没打卡，下班打卡的情况。
select post
,round(sum(unix_timestamp(last_clockin)-unix_timestamp(first_clockin))/3600/count(*),3) work_hours
from staff_tb t1
left join attendent_tb t2
on t1.staff_id=t2.staff_id
where first_clockin is not null
and last_clockin is not null
group by post

order by work_hours desc;