Re: Universal Quantifier

On Jan 27, 1:58 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
> paul c wrote:
> > Marshall wrote:
> > What formula would express a primary key?Faking it heavily, I suggest something along the lines of:
>
> forall A1(p1,q1) in A(p,q). forall A2(p2,q2) in A(p,q).
> if p1 = p2 then q1 = q2;
>
> where p is actually the set of attributes composing the key and q is
> actually the set of dependent attributes.
>
> One also has to express irreducibility, though.

Not really .You cannot and you don't have to.

You cannot because that is a second order concept that is a statement about the set of all valid database instances in the form "for each proper subset of the attribute set there is a valid database instance such that ...", and you seem to be using here a logic that makes statements about individual instances in the form "for each valid database instance it holds that ...".

You don't have to because it would not be a database constraint that the database would have to maintain. In fact, it could not even maintain it if you wanted to because you cannot tell from an individual database instance whether it holds or not.

• Jan Hidders