Re: No exceptions?

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Fri, 30 Jun 2006 22:50:27 GMT
Message-ID: <TUhpg.4317$pu3.101475_at_ursa-nb00s0.nbnet.nb.ca>

Jon Heggland wrote:

[Quoted] > 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.

I respectfully suggest the confusion caused by your use of key without the 'candidate' qualification demonstrates exactly why we have the term. I suppose irreducible key would do just as well, but for historical reasons, candidate key already means an irreducible key. Received on Sat Jul 01 2006 - 00:50:27 CEST