UPDATE DIAPLUSDB.dbo.SalesForecast 
set JUL_SAL = isnull(JUL_SAL,0) + isnull(ss.SLS_DOL,0)
FROM
DIAPLUSDB.dbo.SalesForecast sf
left join SALES_SUMMARY ss on ss.CSCODE = sf.CustID and ss.PLT_NO = sf.PlantID 
WHERE ss.PPPERIOD = '201207' and ss.SLS_DOL is not null

select sf.CustID, sf.PlantID, sf.JUL_SAL, ss.CSCODE, ss.PLT_NO, ss.SLS_DOL, ss.PPPERIOD
from DIAPLUSDB.dbo.SalesForecast sf
left join SALES_SUMMARY ss on ss.CSCODE = sf.CustID and ss.PLT_NO = sf.PlantID 
WHERE ss.PPPERIOD = '201207' and ss.SLS_DOL is not null
order by sf.CustID, sf.PlantID

;WITH  SUM_SALES_SUMMARY
AS
(
    SELECT   CSCODE         AS CSCODE
            ,PLT_NO         AS PLT_NO
            ,SUM(SLS_DOL)   AS SUM_SLS_DOL
    FROM    SALES_SUMMARY 
    WHERE   PPPERIOD = '201207' 
)
    
UPDATE  DIAPLUSDB.dbo.SalesForecast 
SET     JUL_SAL = ss.SUM_SLS_DOL
FROM    DIAPLUSDB.dbo.SalesForecast     AS sf
JOIN    SUM_SALES_SUMMARY               AS ss
ON      ss.CSCODE = sf.CustID 
    AND ss.PLT_NO = sf.PlantID 

UPDATE sf
set JUL_SAL = isnull(sf.JUL_SAL,0) + isnull(ss.SLS_DOL,0)
FROM
DIAPLUSDB.dbo.SalesForecast sf
left join (
    SELECT   CSCODE, PLT_NO, SUM(SLS_DOL)   AS Total_SLS_DOL
    FROM    SALES_SUMMARY 
    WHERE   PPPERIOD = '201207' and SLS_DOL is not null
  ) ss  on ss.CSCODE = sf.CustID and ss.PLT_NO = sf.PlantID