# Re: Sixth normal form

Date: Tue, 07 Aug 2007 19:30:00 -0000

Message-ID: <1186515000.990131.285320_at_q75g2000hsh.googlegroups.com>

On 7 aug, 20:36, vldm10 <vld..._at_yahoo.com> wrote:

> On Aug 1, 7:36 am, Jan Hidders <hidd..._at_gmail.com> wrote:

*>
**> > Any attempt to reformulate it to something easier or more intuitive in
**> > my experience almost always ends up with something that is either
**> > wrong or actually harder to understand.
**>
**> > The only somewhat mysterious part may be the "JD is implied by the
**> > CKs" but this can be tested by the following simple procedure:
**>
**> > 1. Let jd be the join dependency we want to test
**> > 2. While jd has two elements (being sets of attributes) Si and Sj such
**> > that the intersection of Si and Sj contains a candidate key do:
**> > 2.1 replace Si and Sj with the union of Si and Sj
**> > 3. If jd contains the header of the relation (which is also a set of
**> > attributes) then return "yes" else "false"
**> > -- Jan Hidders
**>
**> You gave here the procedure which is more on intuitive level than
**> based on some formal system.
*

Although somewhat informally described by me, it is a proper algorithm and as such *is* a formal system that has been proven both sound and complete.

> The other thing here which is maybe with a questionable meaning is

*> "The only somewhat mysterious part...". This seems like there are
**> some other parts in definition of 5NF.
*

You may want to check your irony-meter. It seems broken. :-)

> Now we can set the question - why mentioned procedure for misterios

*> part is combination of formal and intuitive. Although, mentioned
**> procedure is useful, I beleive it is good to be aware of the
**> following:
**>
**> a) Cks are based on FDs and for FDs there is a formal system.
**> b) For JDs there is no formal system in the sense of complete
**> inference rules.
*

JDs have been succesfully axiomatized. In fact, much larger classes including MVDs, FDs and much more have been axiomatized. See the Alice book for a wealth of information on that.

- Jan Hidders