SELECT
  post
  ,round(avg(TIMESTAMPDIFF(second, first_clockin,last_clockin)/3600),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