hi mezo,

It depends on your datamodel where to store changing data. As for example merchandise management systems (also retail systems, ERP) usually distinguish between master data which remain almost unchangeable and transaction data which differs, for example such things like customers order data. If your customers can order a product in different quantities, say 5, 6, or 7 pieces / unit this information should be stored in an extra table, e.g. customers order item table together with item number, total price etc.

On the other hand you usually need a further table (and much more in real world applications) where you keep book which qty/unit can be assigned to a certain product, maybe also data to get total price.

So your core tables could look like these three:
masterdata(ItemNb#, Itemname),
Customerorderitems(orderNb#, ItemNb#, quantity, qty_per_unit, total_price),
productunits(ItemNb#, qty_per_unit#, price). (where # denotes primary key columns).

Maybe I can give you more help, if you show more details of your data model.

btw, it isn't a good idea to add three columns to your product table for recording the given three qty / unit because there would arise some serious problems when trying to create useful SQL queries or also when doing normalization.

krs,
cliff