Re: Relation Schemata vs. Relation Variables

paul c wrote:
> 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.

Okay, maybe now I'm seeing the subtlety, if you are talking about the tuple after it's been identified, ie., a tuple in the context of a relation. If I've got that right, I could have been more clear that I was talking about identifying a tuple within a relation. Your emphasis on language precision, which some people might call pedantry, hurts my head, but I suppose it's necessary and I really shouldn't complain if the theorists are to find any common ground with the hackers (or if the hackers are to get their act together).

p Received on Fri Aug 25 2006 - 02:59:48 CEST