select
    pay_ability,
    concat (
        round(
            count(distinct overdue_days) / count(distinct lt.customer_id) * 100,
            1
        ),
        '%'
    ) as overdue_ratio
from
    loan_tb lt,
    customer_tb ct
where
    lt.customer_id = ct.customer_id
group by
    pay_ability
order by
    overdue_ratio desc;