Re: Little design mistakes that can be easily avoided (2): Listenning to CELKO (and CELKO alikes)
From: Jon Heggland <jon.heggland_at_idi.ntnu.no>
Date: Mon, 28 May 2007 11:44:34 +0200
Message-ID: <f3e8cu$s81$1_at_orkan.itea.ntnu.no>
>
> I differ with the wording above. Here's an alternate wording: if you have
> a relation, the tuples will be distinct, by definition. This means that the
> entire tuple is either a candidate key or a super key.
Date: Mon, 28 May 2007 11:44:34 +0200
Message-ID: <f3e8cu$s81$1_at_orkan.itea.ntnu.no>
David Cressey wrote:
> "Matthias Klaey" <mpky_at_hotmail.com> wrote in message
> news:5bsi53h8bt88nf7iaj0tpk6e3rbjkv9mh8_at_4ax.com...
>> 1. In the Relational Model, the key is part of the definition of a >> relation (= table in practice). You don't have a relation if it >> doesn't have a key.
>
> I differ with the wording above. Here's an alternate wording: if you have
> a relation, the tuples will be distinct, by definition. This means that the
> entire tuple is either a candidate key or a super key.
It is /always/ a superkey. It might be "candidate" (i.e. irreducible) as well; if not, it is a /proper superkey/. (Also, a tuple is not a key, but I'll quibble about that some other time:)
-- JonReceived on Mon May 28 2007 - 11:44:34 CEST