select s.post,round(avg(TIMESTAMPDIFF(minute, a.first_clockin, a.last_clockin)/60),3) as work_hours
from staff_tb s
join attendent_tb a
on s.staff_id = a.staff_id
group by s.post
order by work_hours desc

TIMESTAMPDIFF(minute, a.first_clockin, a.last_clockin)/60)