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Editorial Team
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Asked: 2026-05-25T16:26:58+00:00

First, I’d like to start out expressing that I am not trying to just

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First, I’d like to start out expressing that I am not trying to just have someone create my table schema for me. I have spent some time weighing the options between the two possibilities in my design and I wanted to get some advice before I go and run wild with my current idea.

Here is my current schema, I will put a ? next to columns I’m considering using.

Key:

table_name
----------
col1 | col2 | col3

tax_zone
---------
tax_zone_id | tax_rate | description

sales_order
-----------
sales_order_id | tax_zone_id (FK) or tax_rate (?)

sales_order_item
-----------
sales_order_item_id | sales_order_id (FK) | selling_price | amount_tax or tax_rate (?)

So, if it wasn’t already clear, the dilemma is whether or not I should store the tax data in the individual rows for an order, or use a join to pull the tax_zone information and then do something in my query like (tz.tax_rate * so.order_amount) as order_total.

At present, I was thinking of using the method I just described. There is a problem I see with this methodology though that I can’t seem to figure out how to remedy. Tax rates for specific zones are subject to change. This means that if a tax rate changes for a zone and I’m using a foreign key reference, the change in the rate will reflect in past orders that were done with a different rate. This causes an issue because at present I’m using the data in this table to store both orders that have been processed and orders that are still open, therefore if someone were to go re-print a past order, the total amount for the order will have changed.

My problem with storing the specific rate or tax amount is that it means every time someone was going to edit an order, I would have to update that row again with the changes to those values.

In the process of writing this, I’m starting to move towards the latter idea being the better of the two.

Perhaps if someone can just provide me the answer to the following questions so I can go research them myself some more.

Is this a known problem in database modeling?
Are there any well known “authorities” on the subject that have published a book / article?

Any help is much appreciated, thanks!

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1 Answer

  1. Editorial Team

    Well, versioning and history is a well known problem in database modelling. Your solution is very common.

    For a simple enumeration like VAT-rates a simple “foreign key tax_id referencing taxtable(id)” will do. The tax-table should never be updated, once a tax_id is enterered, it should stay there forever. If the tax rates are changed at the end of the year, new record should be entered into the tax_table even if records with the new value already exist.

    The best search phrase for search engines is probably “temporal database”.

    UPDATE:
    http://www.google.nl/url?sa=t&source=web&cd=2&ved=0CCMQFjAB&url=http%3A%2F%2Fwww.faapartners.com%2Fdownloads%2Foverige-publicaties%2Fpresentatie-over-tijd-in-databases%2Fat_download%2Ffile&rct=j&q=veldwijk%20temporal&ei=HQdxTtimCcKr-QansM28CQ&usg=AFQjCNEg9puU8WR1KIm90voSDp13WmE0-g&cad=rja

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