Generally, you would not do that. That stores the same data in two
different tables and is redundant. It violates "normalization rules".
There are cases where you do want to do this. You may want to store the
cost of an item in a table and change it every day or so. In your related
(transaction) tables, it would make sense to store the cost of the item at
the time you performed a transaction. I can't think of a similar reason to
store customer data in a related table though. The customer data does not
change very often, and I can't think of any variable data for a customer
you'd want to store with a related table. If you are selecting the customer
number and want the customer name to be stored in the related table, for
example, then you are doing it wrong.
Post a specific example and perhaps we can help you "normalize" the data
better. In general, a piece of data should be stored only once.
