Re: boolean datatype ... wtf?

From: Brian <brian_at_selzer-software.com>
Date: Fri, 1 Oct 2010 15:39:31 -0700 (PDT)
Message-ID: <80464ad5-cc11-49c3-8777-1744b9e4f4f4_at_x42g2000yqx.googlegroups.com>


On Oct 1, 2:26 pm, Erwin <e.sm..._at_myonline.be> wrote:
> On 1 okt, 19:54, Brian <br..._at_selzer-software.com> wrote:
>
>
>
>
>
>
>
> > > Erwin wrote:
>
> > > > Explain "cyclical referential constraint".
>
> > > > And explain "additional attribute".
>
> > The functional dependency AB -> CD is lost.  If that dependency were
> > invalid in the 5NF schema, {A,B,C,D} KEY {A,B}, then it would not be
> > in 5NF because that dependency is implied by the key, so the correct
> > 5NF schema would be,
>
> > {A,B,C} KEY {A,B}, {A,B,D} KEY {A,B},
>
> > which just happens to also be in 6NF.
>
> > In order for the functional dependency AB -> CD to be preserved, a
> > cyclical inclusion dependency
>
> > {A,B,C}[A,B] = {A,B,D}[A,B]
>
> If you care to explain, my question amounted to "why do you qualify
> this as 'cyclical' ?".
>
> And you also spoke of "additional attribute".  Please point out to me
> where this "additional attribute" is in your very own answer ...- Hide quoted text -
>
> - Show quoted text -

Suppose you have a 5NF schema R {A,B,C,D,E} KEY {A,B}

The inclusion dependency equivalent 6NF schema,

Rc {A,B,C} KEY {A,B},
Rd {A,B,D} KEY {A,B},
Re {A,B,E} KEY {A,B},

Rc[AB] = Rd[AB] AND Rc[AB] = Re[AB]

can also be written

Rc[AB] IN Rd[AB] AND Rd[AB] IN Re[AB] AND Re[AB] IN Rc[AB]

Notice the cyclical nature of the constraint:

Rc references Rd, Rd references Re and Re references Rc

When there are only two relation schemata, the cyclical nature of the constraint doesn't stand out like it does when there are three or more.

In this example, there is an additional attribute in the 5NF relation schema then in the example I used in my last post. Note that no additional constraints are needed that aren't implied by the key. Notice further that the equivalent 6NF schema has not only an additional relation schema, Re {A,B,E} KEY {A,B}, but also an additional cyclical constraint, Rc[AB] = Re[AB]. Received on Sat Oct 02 2010 - 00:39:31 CEST

Original text of this message