Re: Sixth normal form

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.
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.
The sentence "JD is implied by CKs" is main and only part of 5NF definition (if we don't analyze trivial cases). 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:

1. Cks are based on FDs and for FDs there is a formal system.
2. For JDs there is no formal system in the sense of complete inference rules.
3. In 5NF, JDs are based on CKs

Of course, in my opinion, 5NF is important result.

Vladimir Odrljin Received on Tue Aug 07 2007 - 20:36:40 CEST