The basic principle is that the current stock level per item is the sum of all stock acquisitions (purchase orders) of the item less the sum of all stock disposals (sales orders and other disposals). You might like to take a look at Inventory.zip
in my public databases folder at:
https://onedrive.live.com/?cid=44CC60D7FEA42912&id=44CC60D7FEA42912!169
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This little demo file illustrates some of the basic mechanisms for inventory management and includes, in addition to sales and acquisitions of stock, provision for non-sale disposals of stock and periodic stock takes.
Ken Sheridan, Stafford, England