create summaries and update a second table?

Hello *,

I have many tables (one per product) and the are two columns where I
put my buyed and selled pieces.  Second I have a Main-Table with all
products and a summary of the stock.

What I want is, whenever I buy or sell something, the changements
(buy/sell) in the product table must triger an update in the Main-
Table.

I am puzzeling arround, how can this be done.

Any help welcome

Thanks
Michelle

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