Say I had two tables and that each one had, within it, a common
primary key (stock_num).

eg.

products_in_stock:
stock_num
quantity

products_ordered_for_stock:
stock_num
quantity

I'd like to be able to select all the stock_num's in products_in_stock
that aren't in products_ordered_for_stock. Any idea as to how I might
do this?

I realize the database could redone more efficiently (maybe just
making one table with three rows - stock_num, quantity_in_stock,
quantity_ordered), but unfortunately, that's not an option.

Any ideas?