Re: Relation Schemata vs. Relation Variables
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Fri, 25 Aug 2006 00:30:06 GMT
Message-ID: <iwrHg.1171$9u.16089_at_ursa-nb00s0.nbnet.nb.ca>
>
> No argument about a candidate key being a constraint. I`m talking about
> the value of a candidate key. If you can infer the values of the other
> attributes from that value, I`d say you have achieved identification.
Date: Fri, 25 Aug 2006 00:30:06 GMT
Message-ID: <iwrHg.1171$9u.16089_at_ursa-nb00s0.nbnet.nb.ca>
paul c wrote:
>> paul c wrote: >> >>> paul c wrote: >>> ... >>> >>>> PMFJI, I would say that the VALUE of a candidate key identifies one >>>> and only one tuple FOREVER! >>> >>> Stupid me, I have to take part of that back - the value of a >>> candidate key obviously could identify several tuples but I still >>> think that would hold forever. Might have been better to say the >>> value of a candidate key identifies a tuple regardless of time. >> >> A candidate key does not identify a tuple. A candidate key is a >> constraint on a relvar and not on a tuple.
>
> No argument about a candidate key being a constraint. I`m talking about
> the value of a candidate key. If you can infer the values of the other
> attributes from that value, I`d say you have achieved identification.
And one cannot infer anything from a subset of the attributes when one is talking about a tuple. The only thing that identifies a tuple is the tuple's value. Just as the only thing that identifies the number 5 is the number 5. Received on Fri Aug 25 2006 - 02:30:06 CEST