select
    ct.pay_ability as pay_ability,
    CONCAT(round(sum(case when lt.overdue_days is not null then 1 else 0 end) / count(*)*100,1),'%') as overdue_ratio
from
    loan_tb lt 
join
    customer_tb ct on lt.customer_id = ct.customer_id
group by
    ct.pay_ability
order by
    overdue_ratio desc
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