Re: Multiple tables refer to one -To use foreign keys or not?

From: Roy Hann <specially_at_processed.almost.meat>
Date: Fri, 30 May 2008 10:11:12 +0100
Message-ID: <>

"Adriano Varoli Piazza" <> wrote in message
> As one of my first encounters with SQL (The RDBMS used is MySQL 5.1),
> my colleague and I have designed the following (it's a part of a
> catalog system for a museum):
> We have an "attachments" table, which can refer to "works_of_art",
> "people", "reproductions" (Let's say, more than three tables). So a
> work of art can have attachments, a person can have a different
> attachment, etc.
> To be able to relate each object to its eventual attachments, one
> solution I imagined is to put N foreign keys from each of the tables
> referred in attachments (if my use of the verb refer is incorrect,
> please do point it out). Let's say that, for now, there are no fields
> exclusive to any of the referred tables, so creating different
> attachment tables is not the best obvious choice.
> The alternative solution we came up with was to use two fields in
> "attachments" to identify the relations: one with the id of the
> referred table, and one with the "type". So 'works of art' would be 0,
> 'people' would be 1... The reasoning behind the decision was that my
> colleague felt that too many fields would remain empty with the first
> solution, and that adding a new type of "attachee" was simpler/more
> efficient than adding a new foreign key.
> This solution doesn't strike me as the best. We want to use cakephp to
> build the app that will use the database, and while this solution
> would eventually work with some manual labour, I feel it grates
> against it when the rest of the design does not. Since my knowledge of
> SQL is tenuous at best, I wanted to listen to some informed opinions
> about this before arguing again.
> By all means, if this is a WTF, point it out, the reason, and possibly
> an alternative.

If I understand your intention correctly then I am sorry to say it probably is a WTF. I say "probably" because you actually describe two intentions. In one place you talk about relating objects (sic) to attachments, and immediately after that you talk about doing the opposite.

I suspect you are uncomfortable with the idea of multiple attachment tables because superficially they would look the same (in the sense they'd have the same column names, each of the same type). That is a programmer's point of view. To a database designer they should appear to be VERY different and I suspect that if you look deep into the logic of what one is justified in doing with attachments for people versus attachments for works_of_art, those differences will become apparent. Once you see that you will be perfectly happy to have multiple attachment tables. The physical model would correspond closely to the logical model.

That makes life easy because it makes it hard to write a correctly formed query that executes correctly and yet gives a wrong or meaningless answer. Database design is not about making it easy to do correct things so much as it is about making it hard to inadvertently do wrong things, and rather paradoxically (it seems to me) that often ends up making correct things easy into the bargain.

Having offered an opinion, I ought to say you'd be unwise to follow any specific advise you get from a newsgroup. We all have our own enthusiasms, prejudices, and preconceptions of what you are trying to do.

Roy Received on Fri May 30 2008 - 11:11:12 CEST

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