Isso é tão fácil quanto
IsNull(FieldName, 0)
Ou mais completamente:
SELECT iar.Description,
ISNULL(iai.Quantity,0) as Quantity,
ISNULL(iai.Quantity * rpl.RegularPrice,0) as 'Retail',
iar.Compliance
FROM InventoryAdjustmentReason iar
LEFT OUTER JOIN InventoryAdjustmentItem iai on (iar.Id = iai.InventoryAdjustmentReasonId)
LEFT OUTER JOIN Item i on (i.Id = iai.ItemId)
LEFT OUTER JOIN ReportPriceLookup rpl on (rpl.SkuNumber = i.SkuNo)
WHERE iar.StoreUse = 'yes'