From: Jan Hidders <hidders_at_gmail.com>
Date: Fri, 07 Sep 2007 19:29:07 -0000

[Quoted] On 7 sep, 19:40, "Brian Selzer" <br..._at_selzer-software.com> wrote:
> "Jan Hidders" <hidd..._at_gmail.com> wrote in message
>
>
>
>
> > On 7 sep, 15:43, Jon Heggland <jon.heggl..._at_idi.ntnu.no> wrote:
> >> Quoth Evan Keel:
>
> >> > "David Portas" <REMOVE_BEFORE_REPLYING_dpor..._at_acm.org> wrote in
> >> > message
> >> >news:pZSdnfih_oRy5H3bRVnyigA_at_giganews.com...
> >> >> Call me picky but I am not entirely comfortable with that U.Texas
> >> >> site,
> >> >> even though it's a commendable effort overall.
>
> >> >> "The definition of second normal form states that only tables with
> >> >> composite primary keys can be in 1NF but not in 2NF"
>
> >> > They have it right here. All non-key attributes must be dependent on
> >> > the
> >> > full key. Only applies to tables with keys composed of multiple
> >> > columns>
>
> >> No. There may be dependencies where the left side is empty.
>
> > In that case the declared candidate key is not really a candidate
> > key. Under the assumption that the declared candidate keys are indeed
> > candidate keys, the claim is correct.
>
> That's not true, Jan, consider:
>
> {A, B, C} | A --> B /\ {} --> C
>
> If {} --> C holds, then A --> C also holds.
> Since A --> B /\ A --> C, then A --> BC.
>
> {{A:1, B:2, C:1}
> {A:2, B:4, C:1}
> {A:3, B:4, C:1}
> {A:4, B:3, C:1}}
>
> Clearly A is the only candidate key, even though the FD {} --> C holds.

Indeed, I stand corrected.

• Jan Hidders
Received on Fri Sep 07 2007 - 21:29:07 CEST

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