Re: Universal Quantifier

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Sun, 28 Jan 2007 13:27:11 GMT
Message-ID: <Pw1vh.6169$1x.107942@ursa-nb00s0.nbnet.nb.ca>

Jan Hidders wrote:

>
> 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.

Cool. That lets us off the hook then. Received on Sun Jan 28 2007 - 07:27:11 CST