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@ursa-nb00s0.nbnet.nb.ca>

paul c wrote:

> Bob Badour 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 Thu Aug 24 2006 - 19:30:06 CDT