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Re: No exceptions?

From: Jon Heggland <jon.heggland_at_idi.ntnu.no>
Date: Sat, 01 Jul 2006 00:08:14 +0200
Message-ID: <e847g8$ero$1@orkan.itea.ntnu.no>


J M Davitt wrote:
> Jon Heggland wrote:

>> 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.

>
> I understand your point about candidate keys; I believe the term
> is used to distinguish the irreducible set of attributes that hold
> unique values from the sets of values that comprise superkeys.

In my experience, it is used to distinguish "primary" keys from the rest. No matter.

> The rest of your post confuses me. I understand that the last
> sentence refers to a set of keys, but I'm not sure how it is that
> there is only one key in that set of keys if that key's set of
> attributes is empty. It seems to me that every attribute would be
> a key and every combination of attributes would be a superkey. I
> don't see how an empty attribute key precludes other keys.

"Every attribute" would also be a superkey (speaking loosely). The empty set is a subset of every set.

-- 
Jon
Received on Fri Jun 30 2006 - 17:08:14 CDT

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