The relational db advantage that is after normalization data is not
duplicated like in old flat-file days. But I am trying to understand how
I would design tables to preserve old record states. An example will
better illustrate my question.

Say you invoice table and items can have 3 tax-types merchandise, food,
and tax-exempt. So you have invoice table with a foreign key to a tax
table with description and rates, 5%, 2%, 0% respectively. So the state
legislature changes the rates effect July 1st to 6%, and lowers food to
1%. Change the tax table and great, all orders now will have the latest
rates, but if you look up an older invoice before the rate change then
those records will be incorrect. How best to design tables such that it
will preserve old records without resorting to old methods; the PPC
method (Print Paper Copy ;-) ), or flat-file method where you just
duplicate the tax type and rates in the one table?

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Take care,

Jonathan
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http://www.LittleWorksStudio.com