Re: Multiple keys and transition constraints
Date: Fri, 15 Sep 2006 03:21:17 GMT
Message-ID: <N_oOg.42$8e5.29_at_tornado.ohiordc.rr.com>
Brian Selzer wrote:
> "JOG" <jog_at_cs.nott.ac.uk> wrote in message
> news:1158251660.983027.98520_at_e3g2000cwe.googlegroups.com...
>
>>Brian Selzer wrote:
>>
>>>Given a relation schema R {A, B, C}, where A and B are each candidate
>>>keys.
>>>
>>>If the current extension is
>>>
>>>r {{A:1, B:9, C:3}
>>> {A:2, B:8, C:4}}
>>>
>>>and the proposed extension is
>>>
>>>r' {{A:1, B:8, C:4}
>>> {A:2, B:9, C:3}}
>>>
>>>is only A different? Or are both B and C different? From one
>>>perspective,
>>>both B and C remain constant but A is different. From another
>>>perspective,
>>>A remains constant but both B and C are different. If you're trying to
>>>specify the allowable transitions for C, which key do you use? A, B, the
>>>superkey, AB, or none of the above?
>>
>>This requires more information to answer.
>>
>>Can you tell us which attribute identifies the subject [of each
>>proposition] in real life over the updates. Then I can determine /what/
>>exactly has experienced the transition.
>>
>>No other questions, just that.
>>
> > > Well, that's the dilemma, there are two subjects because there are two keys. > Consider the following relation:
[Why do you flog yourself with this nonsense?!]
"subjects." "keys." Hmm... re: r v. r':
B is a key, you say? Well, the same determinant values in B have different dependent values. So?
A is a key, you say? Well, the same determinant values in A have different dependent values. So?
If that design doesn't make you happy, concoct one that will. Received on Fri Sep 15 2006 - 05:21:17 CEST