select
post
,sum(timestampdiff(minute,a.first_clockin,a.last_clockin)/60.0)/count(distinct s.staff_id) as work_hours
from staff_tb s
join attendent_tb a
on s.staff_id = a.staff_id
where a.first_clockin is not null and a.last_clockin is not null
group by 1
order by work_hours desc

时间差函数函数