Re: Relation Schemata vs. Relation Variables

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 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 1 certainly identifies that tuple.

p
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Relevant Pages

  • Re: Relation Schemata vs. Relation Variables
    ... one and only one tuple FOREVER! ... value of a candidate key identifies a tuple regardless of time. ... Perhaps I should have said "relvar that contains a tuple". ... state t before the transition and the state t' after the transition. ...
    (comp.databases.theory)
  • Re: Relation Schemata vs. Relation Variables
    ... Bob Badour wrote: ... one and only one tuple FOREVER! ... value of a candidate key identifies a tuple regardless of time. ... constraint on a relvar and not on a tuple. ...
    (comp.databases.theory)
  • Re: Relation Schemata vs. Relation Variables
    ... one and only one tuple FOREVER! ... 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. ... A candidate key is a constraint on a relvar and not on a tuple. ... This is a precise field. ...
    (comp.databases.theory)
  • Re: Relation Schemata vs. Relation Variables
    ... one and only one tuple FOREVER! ... Might have been better to say the value of a candidate key identifies a tuple regardless of time. ... 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. ... 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. ...
    (comp.databases.theory)
  • Re: Relation Schemata vs. Relation Variables
    ... and only one tuple FOREVER! ... Might have been better to say the value of a candidate key identifies a tuple regardless of time. ... A candidate key is a constraint on a relvar and not on a tuple. ... If you can infer the values of the other attributes from that value, I`d say you have achieved identification. ...
    (comp.databases.theory)