UPDATE BILL_DETAIL SET GROSSAMT = T1.F1 FROM (SELECT SUM(ITEMAMOUNT) AS F1, BILLNO AS F2 FROM BILL_ITEM_DETAIL GROUP BY BILLNO) AS T1 WHERE T1.F2 = BILLNO
UPDATE BILL_DETAIL SET NETAMT = GROSSAMT 
UPDATE BILL_DETAIL SET NETAMT = GrossAmt+G1.L1 FROM (SELECT SUM(T1.F1) AS L1, T1.F2 AS L2 FROM (SELECT Sum(FldAmount) AS F1, FormSrNo AS F2 FROM FIRM_ADDLESS_DETAIL WHERE AccEff='NO' AND FormName='SALE BILL' AND FldAmount<>0 AND FldAddLess='+' GROUP BY FormSrNo UNION ALL SELECT -Sum(FldAmount) AS F1, FormSrNo AS F2 FROM FIRM_ADDLESS_DETAIL WHERE AccEff='NO' AND FormName='SALE BILL' AND FldAmount<>0 AND FldAddLess='-' GROUP BY FormSrNo) AS T1 GROUP BY T1.F2) AS G1 WHERE G1.L2 COLLATE DATABASE_DEFAULT=BillNo