题解 | 统计各岗位员工平均工作时长

select
    post,avg(round(TIMESTAMPDIFF(MINUTE,first_clockin,last_clockin)/60,3)) as work_hours
from
    staff_tb 
    natural join attendent_tb 
group by
    post
order by
    work_hours desc

sql执行顺序，from➡️where➡️group by➡️having➡️select➡️distinct➡️order by➡️limit/offset

思路：自然连接两个表，根据post分组，然后select得到每组的平均工作时间（TIMESTAMPDIFF函数，不能用timediff，它会把时间直接变成数字，不做转换）