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Re: No exceptions?

From: Jon Heggland <jon.heggland_at_idi.ntnu.no>
Date: Fri, 30 Jun 2006 10:08:46 +0200
Message-ID: <e82ma8$kau$1@orkan.itea.ntnu.no>


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?
> 
> [...] 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?

There is some confusion here on both parts, I think. Any relvar can have an empty key, regardless of the number of attributes in the relvar. It follows that such a relvar can have no other keys. A relvar has a set of (candidate, if you will, but I consider that term meaningless) keys, in general, but this set cannot be empty---there is always at least one key.

-- 
Jon
Received on Fri Jun 30 2006 - 03:08:46 CDT

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