Re: No exceptions?

From: paul c <>
Date: Fri, 30 Jun 2006 05:53:12 GMT
Message-ID: <c%2pg.109999$Mn5.1578_at_pd7tw3no>

paul c wrote:
> J M Davitt wrote:

>> ...
>> All the attributes in a relation comprise, at least, a superkey.  The
>> set of attributes that qualify as a candidate key must hold unique 
>> values and no subset of those attributes must hold unique values.  The
>> only relations that could have empty candidate keys are those with
>> empty headings, right?  

> Apologies - I'll try to read the rest tomorrow, but right now I'm tired
> (from watching a library video - "Guns, Germs and Steel" - the author
> seems to be the James Burke of biology, not that I know much about the
> latter). I thought a relation with any number of attributes could have
> only one value for them if it had an 'empty' set of candidate keys, eg.
> a relation that has only one tuple? (and a 'relvar' could have only one
> value at any instant in time)?
> p

sorry, i think i got that wrong - will try again next day, drinking tea only.

p Received on Fri Jun 30 2006 - 07:53:12 CEST

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