select c.pay_ability,
concat(round(sum(case
when l.overdue_days is not null then 1
else 0
end)/count(*)*100,1),'%') as overdue_ratio

from loan_tb l

inner join customer_tb c

on l.customer_id = c.customer_id

group by c.pay_ability

ORDER BY
    overdue_ratio DESC

感觉 case when用的相当不熟练，需要加强练习