Updating a Record using a current record

[MRW]

Hello,

I'm trying to make an inventory database. When a product is being
shipping, I want to take the current QTY and minus 1 (for example) for
each product being shipped.

I know I can connect to the database by ProductID, use a reader, grab
the QTY, close the reader, do QTY=QTY-1, then send an update query for
all products in that cart.

However, I'm sure there is an easier way of doing this. Does anybody
have any suggestions?

Thanks for any help!