方法一：ifnull()

with base_tb1 as(
select s.department,count(*) as total
from staff_tb s inner join attendent_tb a on s.staff_id = a.staff_id
group by department),
base_tb2 as (
select s.department,case when count(*) =null then 0 else count(*) end as fenzi
from staff_tb s inner join attendent_tb a on s.staff_id = a.staff_id
where round(timestampdiff(minute,a.first_clockin,last_clockin)/60,1)>9.5
group by department
)

select b1.department, concat(round(ifnull(b2.fenzi,0)/b1.total*100,1),'%') as ratio
from base_tb1 b1  left join base_tb2 b2  on b1.department = b2.department