select c.pay_ability,
concat(round(count(case when l.overdue_days is not null then l.customer_id else null end)/count(l.customer_id)*100,1),'%') as overdue_ratio

from loan_tb l 
join customer_tb c on l.customer_id = c.customer_id
group by c.pay_ability
order by overdue_ratio desc

记住 concat(round(count(case when l.overdue_days is not null then l.customer_id else null end)/count(l.customer_id)*100,1),'%')