SELECT (CASE
        WHEN DATEDIFF(NOW(), bill_date) BETWEEN 0 AND 15 THEN '0-15'
        WHEN DATEDIFF(NOW(), bill_date) BETWEEN 16 AND 30 THEN '16-30'
        WHEN DATEDIFF(NOW(), bill_date) BETWEEN 31 AND 45 THEN '31-45'
        WHEN DATEDIFF(NOW(), bill_date) BETWEEN 46 AND 60 THEN '46-60'
        WHEN DATEDIFF(NOW(), bill_date) BETWEEN 61 AND 90 THEN '61-90'
        WHEN DATEDIFF(NOW(), bill_date) BETWEEN 91 AND 180 THEN '91-180'
        WHEN DATEDIFF(NOW(), bill_date) BETWEEN 181 AND 365 THEN '181-365'
        WHEN DATEDIFF(NOW(), bill_date) >= 366 THEN '> 365
            END
        ) as bill_range,
        SUM(pending_amount) as amount_due,
FROM
    `user`
    GROUP BY bill_range