I did read your article and my solution is similar but there are differences - mainly that I'm joining to sub query that is grouping - where you originally are...

Good article.. Thanks!..

Here is how I would have solve the problem.

UPDATE a

SET a.netInventoryQty=c.inventoryQty-b.cQty

--SELECT *

FROM demand a

INNER JOIN (

select a.demandID, sum(b.orderQty) as cQty

FROM demand a

INNER JOIN demand...