Simple Operations of Fields Between Tables

[JCarp2000]

I am doing an inventory database for a small shop. I have one table for my inventory currently in stock. I have two other tables for customer purchases and purchases the store makes to restock. I am wondering how to make it so that if a new customer purchase is made it subtracts from the current inventory and if the store restocks it adds to the current inventory. I know this is a bit confusing so let me know if any clarification is needed.

[pbaldy]

Allen explains why you shouldn't try:

http://allenbrowne.com/AppInventory.html

I would just calculate from transactions.