Re: The Practical Benefits of the Relational Model

"Paul Vernon" <paul.vernon_at_ukk.ibmm.comm> wrote in message news:<aooo8t$1h9s$1_at_sp15at20.hursley.ibm.com>...
> Now it might be a bit of a pain to have a RDBMS that did not allow two
> tables to have the same attribute names (and types) in the same *database*,
> but frankly I could live with such a restriction if it enforced the
> Orthogonal Design Principle (and if any local relvars we not seen as part of
> the main database)

I was thinking of something related to this from a slightly diferent angle: relational theory is built on first-order predicate logic but also I've often heard it said that it is also based on set theory. Now set theory is built on first order predicate logic so I'm not sure if relational theory *requires* set theory or maybe just a weak subset of it or what.

So the relations are the sets and the tuples are the elements of that set.

But one of the axioms of set theory (extension) says that two sets with the same elements are equal. But you could easily have two relations with identical tuples but different "meanings". For example consider a relation with columns in domains "person" and "country". One could be from the predicate "lives in" e.g. "Fred" lives in "the UK". Another with identical domains and tuples could be from the predicate "was born in".

It might just happen that for the universe of people at one particular snapshot that everyone is living in their country of birth.

Maybe there is a need to distinguish between equality and identity here i.e. two relations can be "equal" for a database snapshot but not "identical" or tautologically equivalent for all possible values of the database?

Paul. Received on Tue Oct 22 2002 - 14:58:46 CEST