Re: Multiple keys and transition constraints

From: J M Davitt <jdavitt_at_aeneas.net>
Date: Fri, 15 Sep 2006 03:21:17 GMT
Message-ID: <N_oOg.42$8e5.29@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 Thu Sep 14 2006 - 22:21:17 CDT