Re: constraints in algebra instead of calculus

On May 20, 9:08 pm, paul c <toledobythe..._at_oohay.ac> wrote:
> Marshall wrote:
> > Okay, a while back we were talking about writing constraints
> > in a language with aspects of the relational calculus, specifically
> > the existential and universal quantifiers. The point was made
> > that that's unnecessary; the calculus is no more expressive
> > than the algebra.
>
> > So it ought to be possible to write any constraint from the
> > calculus in the algebra.
>
> > Well, I'm having a hard time figuring out how to do it. Can
> > anyone help?
>
> > How does one write a functional dependency in the algebra?
> > A foreign key?
>
> > Marshall
>
> I thought that one was easy, if FK is the set of 'foreign key'
> attributes and A is the 'referencing table' and using an op like D&D
> <AND>, then it's something like A{FK} <AND> B{FK} = A{FK}. (I like this
> one because it's particularly easy to implement.)-

FK is easy indeed. Assuming that we rename attributes of A and B in such a way that x becomes a common FK attribute, a FK constraint is written in relational lattice as

A \/ X < B \/ X

where X is the empty relation with attribute x. Naturally, I'd suggest too call the constraint between arbitrary 3 relations

A \/ C < B \/ C

as a generalized FK... Received on Mon May 21 2007 - 20:35:34 CEST