Re: Advanced SQL
Date: Fri, 07 Sep 2007 14:19:18 GMT
Message-ID: <G5dEi.2$cy3.0_at_trndny03>
"Jan Hidders" <hidders_at_gmail.com> wrote in message
news:1189173585.627344.49900_at_19g2000hsx.googlegroups.com...
> 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.
>
I'm really confused by the above. First off, if there were a dependency where the left side is empty, wouldn't the same value have to exist the dependent column, for all of the rows?
> > But it's a
> > common mistake. If I remember correctly, one of my database textbooks
> > "proves" that any relvar with just two attributes is automatically in
BCNF.
>
> Under the assumption that I just mentioned, that claim is actually
> also correct.
>
> -- Jan Hidders
> Received on Fri Sep 07 2007 - 16:19:18 CEST