Lógica de relatório do razão no procedimento armazenado

SELECT Patient_nbr,
       billno,
       billamount,
       PAID_AMOUNT,
       CASE
         WHEN RNO > 1 THEN Sum(billamount - PAID_AMOUNT)
                             OVER(
                               PARTITION BY Patient_nbr
                               ORDER BY RNO)
         ELSE Iif(( billamount - PAID_AMOUNT ) < 0, 0, billamount - PAID_AMOUNT)
       END
FROM   (SELECT *,
               Row_number()
                 OVER(
                   PARTITION BY Patient_nbr
                   ORDER BY Patient_nbr) AS RNO
        FROM   #Patient_ledger) A