with shichang as(
  select staff_id,first_clockin,last_clockin,
  TIMESTAMPDIFF(MINUTE, first_clockin, last_clockin)/60  as gongzuo
from attendent_tb)

select post, round(avg(gongzuo),3)as work_hours
from shichang join staff_tb
on shichang.staff_id=staff_tb.staff_id
group by post 
order by work_hours desc

1 计算每个人的工作时长，用timestampdiff，以分钟为单位，再除以60得到小时

2 按职位分组，取avg即可，注意三位小数，排序问题