let me try this again

;

WITH newtable

AS ( SELECT StoreProdID

, sp.ListID

, ProdID

, Qty

, PrevQty

, GroupID

, StoreID

FROM #StoreProducts sp

JOIN #StoreList sl

ON sl.ListID = sp.ListID

)

SELECT tw.ListID

, tw.ProdID

, tw.Qty

, pw.qty AS PrevQty

FROM newtable tw

JOIN newtable pw

ON pw.ProdID = tw.ProdID

AND pw.StoreID = tw.StoreID

AND tw.GroupID - 1 = pw.GroupID