select department,concat(round(sum(times)/count(department)*100,1),'%') ratio from
(select department,if(timestampdiff(minute,first_clockin,last_clockin)>(9.5*60),1,0) times from
staff_tb st join attendent_tb at on st.staff_id=at.staff_id) as A
group by department
order by ratio desc

重要函数：timestampdiff 两个时间点之间的差额，可以体现为第一个参数里【hour,minute,second】

round （字符串，保留的小数点后几位）