I made a mistake, it is in the ON statement that must be conditional, and only on that one field. The data is too messy to get a good sample, but here is the code

SELECT p.Warehouse, p.PartNo, COUNT(w.WONo) AS OrderCount, SUM(w.Qty) AS QtySold, MAX(EntryDate) AS LastSaleDate, WONo

FROM (SELECT WONo, EntryDate, Qty, BillTo, PartNo, PartNoAlias

FROM woparts

WHERE Left(billto,1)='t' and EntryDate > '2012-10-16' and WONo<910000000) w

right join

(SELECT *

FROM parts

WHERE Left(Warehouse,1)='t' and OnHand > 0) p

ON (p.partno=w.partno or w.PartNo = p.PartNoOriginal or w.partno = p.PartNoAlias or w.PartNo = p.OldNUmber or if(p.PartNoAlias='',,w.PartNoAlias=p.PartNoAlias)) and p.Warehouse= left (w.BillTo,5)

GROUP BY p.Warehouse, p.PartNo, WONo