Re: Universal Quantifier

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Sat, 27 Jan 2007 16:01:07 GMT
Message-ID: <7HKuh.5831$1x.100757@ursa-nb00s0.nbnet.nb.ca>

paul c wrote:

> Bob Badour 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.

>
> Regarding irreducibility, do we not express it by our choice of p1 and
> p2? Ie., what we express with a reducible p1 is extraneous?

But one can still express that using not exists and some proper subset of P. Received on Sat Jan 27 2007 - 10:01:07 CST