select post,round(avg(timestampdiff(minute,first_clockin,last_clockin))/60,3) work_hours
from staff_tb t1 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;
#答案错误，错哪了？-以小时为单位，不是以小时为差