SQL & PL/SQL

Dear Seniors

I want to add column in this query sum_amount( that it sum the amount on PUR_NO......when PUR_NO changed them sum must be changed )

thanks

SELECT A.PUR_NO, A.PUR_DATE, A.DISCOUNT,  A.BARCODE, A.ITEM_CODE,

        A.ITEM_NAME, A.QTY, A.UOM, A.RATE, A.RATE_WITH_CHARGES, A.FOC_QTY, A.DISCOUNT_ITM, A.AMOUNT

FROM

(SELECT

  A.PUR_NO,  A.PUR_DATE,  A.LPO_NO,  A.REF_NO,A.STORE_CODE, A.DISCOUNT, A.PAID_AMOUNT, A.OTHER_CHARGES,

  B.SRNO, B.BARCODE,  B.ITEM_CODE, INV.ITEM_NAME(B.BARCODE) ||' '||  B.ITEM_SRNO AS ITEM_NAME, B.QTY,

  INV.ITEM_UNIT_BARCODE(B.BARCODE) AS UOM, B.RATE,

    (CASE WHEN A.OTHER_CHARGES IS NULL THEN  nvl(B.RATE,0)

      ELSE

    ((NVL(B.RATE,0) /

      (select sum(nvl(rate,0)) from inv.purchase_c where pur_no = b.pur_no)*100)/

      NVL(B.QTY,0)) * (NVL(A.OTHER_CHARGES,0) / 100) + NVL(B.RATE,0)

      END) AS RATE_WITH_CHARGES,

     (CASE WHEN A.OTHER_CHARGES IS NULL THEN  nvl(B.RATE,0)

      ELSE

    ((NVL(B.RATE,0) /

      (select sum(nvl(rate,0)) from inv.purchase_c where pur_no = b.pur_no)*100)/

      NVL(B.QTY,0)) * (NVL(A.OTHER_CHARGES,0) / 100) + NVL(B.RATE,0)

      END) * NVL(B.QTY,0) - (NVL(B.QTY,0) * NVL(B.DISCOUNT,0))AS AMOUNT,

     B.FOC_QTY, B.DISCOUNT AS DISCOUNT_ITM

     FROM INV.PURCHASE_M A, INV.PURCHASE_C B

WHERE (A.PUR_NO = B.PUR_NO(+)) --AND (A.PUR_NO = 21)

ORDER BY B.SRNO)A;

/****

SELECT SUM(NVL(

      ((DECODE(B.RATE,0,0,B.RATE /

      (select sum(nvl(rate,0)) from inv.purchase_c where pur_no = b.pur_no)*100)/

      NVL(B.QTY,0)) * (NVL(A.OTHER_CHARGES,0) / 100) + NVL(B.RATE,0))

      * NVL(B.QTY,0) - (NVL(B.QTY,0) * NVL(B.DISCOUNT,0)),0))AS AMOUNT

FROM INV.PURCHASE_M A, INV.PURCHASE_C B;

WHERE (A.PUR_NO = B.PUR_NO(+)) AND (A.PUR_NO = 21);**/