Stock Calculation

[IQRARAO]

I ran a gas plant and working on its table


I have two tables
Sale(SID, Quantity, Amount, VehicleNo, Date)
Purchase(PID, Quantity, Amount, VehicleNo, Date)

I want that at every transaction stock is updated. Can you tell Am I need another table stock which update when ever these table update(If yes then how )

OR

I need to add attribute Stock in Sale that will add when ever purchase transaction is done and substract whenever sale transaction is done.

[Micron]

This is inventory control? Perhaps review http://allenbrowne.com/AppInventory.html

Note - don't use reserved words for Access objects (Date).
ReservedWords