The inventory table has a record referencing "hat". The 5 represents the number of hats in the inventory. In which case, the query I provided is not needed.
However, if you change the total number of "hats" in the table, there will be no record of the change. Are you sure you don't want to track how your inventory was altered and when? This will not help your paper trail if this is the case.
If this is the case, simply choose an unbound field to store the amount of inventory to add or reduce and update the 'Amount' field appropriately depending on the item.
Does this help?
Gary
gwinn7