In your case you can use:

;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