Re: Advanced SQL

From: Brian Selzer <brian_at_selzer-software.com>
Date: Fri, 7 Sep 2007 13:40:58 -0400
Message-ID: <K2gEi.1841$ZA5.602@nlpi068.nbdc.sbc.com>

"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.

>

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.

>> 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 - 12:40:58 CDT