SELECT DISTINCT pay_ability,CONCAT(ROUND(COUNT(overdue_days)/COUNT(*)*100,1) ,'%') as overdue_ratio  from loan_tb  l
inner join customer_tb c
on l.customer_id  =c.customer_id 
group by pay_ability
order by overdue_ratio desc;

有逾期行为客户占比 = 有逾期的/总的。

count（字段名）和count（*）区别在于，前者不统计Null的数量，后者把null也算上个数。