# Re: Sixth normal form

Date: Sat, 11 Aug 2007 03:59:54 -0700

Message-ID: <1186829994.330618.293020_at_d55g2000hsg.googlegroups.com>

On Aug 10, 10:41 pm, "Brian Selzer" <br..._at_selzer-software.com> wrote:

*> "Jan Hidders" <hidd..._at_gmail.com> wrote in message
**>
*

> news:1186751333.018671.305210_at_j4g2000prf.googlegroups.com...

*>
**>
**>
**> > On 9 aug, 04:15, "Brian Selzer" <br..._at_selzer-software.com> wrote:
**>
**> >> The closure of the set of functional dependencies
**> >> includes A --> C, which can only be preserved by the inclusion
**> >> dependency,
**> >> {A,B}[B] IN {B,C}[B].
**>
**> > Not necessarily. That depends on your definition of FDs over
**> > attributes in different relations. The usual definition in
**> > normalization theory is that they hold for a schema if they hold for
**> > the natural join of all relations in the schema. In that case the FD
**> > is preserved also without the inclusion dependency.
**>
**> I don't agree with the usual definition. It isn't strict enough, in my
**> opinion.
**>
**> (1) A --> B /and/ B --> C; therefore A --> C.
**> (2) A --> B /or/ B --> C; therefore A -/-> C.
**>
**> (1) is preserved by the IND {A, B}[B] IN {B, C}[B]; (2) is what is without
**> the IND.
**> While it is true that A --> C in {A, B} JOIN {B, C}, without the IND there
**> can still exist values for A that do not determine a value for C.
*

- Jan Hidders