select post, round(avg(timestampdiff(minute,first_clockin,last_clockin))/60,3) as work_hours
from staff_tb 
join attendent_tb using(staff_id)
where first_clockin is not null and last_clockin is not null
group by post
order by work_hours desc

关于本道题题解，关键是找出上下班打卡时间的差异，第一次用timestampdiff(hour,first,last)的时候发现不对，后来仔细观察数据，发现应该用timestamodiff(minute)/以后更准确