Re: Relation Schemata vs. Relation Variables

From: paul c <toledobythesea_at_oohay.ac>
Date: Fri, 25 Aug 2006 00:40:00 GMT
Message-ID: <AFrHg.464472$IK3.372354@pd7tw1no>

Bob Badour wrote:
> 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.

There must be a subtlety here that eludes me. If a candidate key, k, of a tuple has a value of 1 and a tuple in a relation that has that candidate key has a value of (1,2) then I would say that the value k=1 certainly identifies that tuple.

p Received on Thu Aug 24 2006 - 19:40:00 CDT