SELECT
    s.post,
    ROUND(AVG(TIMESTAMPDIFF(minute,a.first_clockin,a.last_clockin)/60),3) AS work_hours
FROM
    staff_tb s 
    JOIN attendent_tb a ON s.staff_id=a.staff_id
WHERE
    a.first_clockin IS NOT NULL
    AND a.last_clockin IS NOT NULL
GROUP BY
    s.post
ORDER BY 
    work_hours DESC

IS NOT NULL代表非空