Thank you all for our assistance. I got a solution from Utter Access message board.
UPDATE DIAPLUSDB.dbo.SalesForecast
SET JUL_FOR = isnull(sf.JUL_FOR,0) +
( SELECT SUM(SLS_DOL)
FROM SALES_SUMMARY
WHERE (SALES_SUMMARY.CSCODE = sf.CustID)
AND (SALES_SUMMARY.PLT_NO = sf.PlantID)
AND (SALES_SUMMARY.PPPERIOD = '201207')
AND (SALES_SUMMARY.SLS_DOL is Not NULL) )
FROM DIAPLUSDB.dbo.SalesForecast sf, SALES_SUMMARY
The structure isn't intuitive to me, but, now I have an understanding of the structure and can construct the same in the future (I think).
Sergiy, you asked, "I'm not sure why do you need LEFT JOIN here."
I guess I don't have a comprehensive understanding of the "mechanics" of the JOIN - what I call 'under the hood'. In my mind the record processing, is to view each SalesForecast record, find matches in SALES_SUMMARY (CSCODE & PLT_NO), but do not include those outside the Period (PPPERIOD) and those with NULL SLS_DOL. One thing I know is that there are fewer SalesForecast records than SALES_SUMMARY.
Anyway, Thanks all again for your input.
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