select pay_ability,concat(round(sum(if(b.overdue=0,0,1))*100/count(*),1),'%') as overdue_ratio  #用字段concat函数进行字段连接，把%加上去
from(
select a.customer_id,pay_ability,overdue_days,if(overdue_days is null,0,1) as overdue
from(
select loan_tb.customer_id,pay_ability,overdue_days
from loan_tb
left join customer_tb
on customer_tb.customer_id=loan_tb.customer_id) a) b
group by pay_ability
order by overdue_ratio desc