>>I also duplicate the sales tax rate in my Invoice table. I have a SalesTax table of current rates, but because they change, I have to have a SalesTax field (2 actually -- an additional one for the Non-Taxable items rate [which is not always 0%. Louisiana actually charges 4% State on tax exempt items]).
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>>In some instances, you would need to repeat people's names in tables other than the Customer, Census, Employee, etc., table if a historical need of name changes (marriage, divorce, religious conversions, NBA players, etc) is necessary.
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>The duplication of Sales Tax rate in Invoice table can be avoided. Give a Unique name to the type of taxation and assing the percentage in a seperate table. Once an Invoice is raised, for that Tax structure, its percentages CANNOT be modified. By this design you can just store the Tax id of tax table in Invoice table

The problem is that cities and other taxing authorities can change the tax rate. Therefore, the need exists to store the taxrate at the time of the transaction be stored with the sale.

I definitely have a sales tax table to store the current tax rate. I just find it easier to duplicate the tax rate in the sales table than keep up with taxrates and effective dates in a sales tax table.