Re: A pk is *both* a physical and a logical object.

Brian Selzer wrote:
...
> Given a choice between a key that permanently identifies individuals and a
> key that contingently identifies individuals, which would you choose to be
> the primary key, and thus the target of all foreign keys? Don't you see a
> problem with a relation that represents the histories of a set of
> individuals where those individuals are denoted only by a set of attributes
> that contingently identify them?
> ...

No, I don't imagine any particular problem. I'm darned if I'm going to get sucked into judging such shapeless questions in a vacuum, whether contingent vacuum or non-contingent vacuum, whatever contingent means here. If the word "contingent" ever has any place in a db topic, even though I doubt it does, it would be with reference to an application. Same goes for other loose lingo like "rigid".

> But requiring that all key values permanently identify individuals
> significantly limits the expressiveness of the model, cutting in half the
> expressions that can be used to denote individuals, thereby reducing the
> number of queries that can be formulated. Referring to my previous example,
> a query such as "Which part has lot number 203 in location 22?" could not be
> considered deterministic if the key, {lot_number, location}, were not
> defined, and thus should be rejected as being formulated incorrectly.
> Without that key, there can be more than one part with lot number 203 in
> location 22, and you can't stuff a relation value into a tuple variable.
> ...

I'm glad that isn't phrased as a question, because words would fail me if I tried to answer it! Maybe somebody else wants to try.

p Received on Mon Jul 30 2007 - 06:44:07 CEST