select post,
-- 计算总分钟数，然后除以 60 得到小时数
  round(avg(TIMESTAMPDIFF(MINUTE, first_clockin, last_clockin) / 60),3) AS work_hours
from staff_tb a
join  attendent_tb b on a.staff_id = b.staff_id
where first_clockin is not null and last_clockin is not null
group by post
order by work_hours desc
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