select pay_ability,concat(round(t1.overdue_num/t2.total_num*100,1),'%') overdue_ratio
from
(select ct.pay_ability,count(lt.overdue_days) overdue_num
from
customer_tb ct join loan_tb lt on ct.customer_id = lt.customer_id
group by ct.pay_ability) t1
join 
(select ct.pay_ability,count(ct.customer_id) total_num
from
customer_tb ct
group by ct.pay_ability ) t2 using(pay_ability)
order by overdue_ratio desc