# Re: Sixth normal form

Date: Mon, 27 Aug 2007 16:30:18 GMT

Message-ID: <u_CAi.51010$YL5.48323_at_newssvr29.news.prodigy.net>

"Brian Selzer" <brian_at_selzer-software.com> wrote in message
news:qoLzi.21288$eY.3721_at_newssvr13.news.prodigy.net...

*>
*

> "David Cressey" <cressey73_at_verizon.net> wrote in message

*> news:4JFyi.3591$wr3.2580_at_trndny04...
**>>
**>> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
**>> news:9NDyi.28722$RX.5655_at_newssvr11.news.prodigy.net...
**>>> begin with. I never really came up with a clear definition of what it
**>> means
**>>> for a schema to have /at least as much/ information, nor did I provide
**>>> any
**>>> proof that if a relation is in 5NF, then the relationships between the
**>>> dependent attributes are due to the fact that the relation is in 5NF, or
**>>> that the presence of a pathological relationship between the dependent
**>>> attributes indicates that the relation isn't in 5NF.
**>>
**>> Part of the problem may be that the phrase "at least as much information"
**>> suggests some sort of measure of information, but not the information
**>> itself. For example, if you have "at least as much money in the bank as
**>> I
**>> have", it doesn't mean that you have the same money as I do in the bank.
**>>
**>> I think what you may have meant might be better conveyed by a phrase like
**>> "at least all the same information as". But I'm not sure what you did
**>> mean,
**>> so this is just a guess.
**>>
**>
**> What I was trying to convey by the phrase /at least as much/ information
**> is that the only additional information that should ever appear in an
**> instance of the more normalized database schema is exactly that
**> information that should be allowed but can't be due to the structure of
**> the less normalized database schema. For example, if the FDs A --> B and
**> B --> C hold in a relation schema {A, B, C}, then it is not possible to
**> insert values for B and C without also inserting a value for A. If it
**> should be possible, then there is a structural problem which is due to the
**> fact that the MVD B ->-> A | C that holds in {A, B, C} is pathological.
**> The problem is that while the decomposition into {A, B} and {B, C} makes
**> it possible to insert values for B and C without also requiring a value
**> for A, it also permits values to be inserted for A and B without also
**> requiring a value for C. The inability to insert a value for A without
**> also inserting a value for C is due to the fact that the FD A --> C holds
**> in {A, B, C}. Even though it is true that the
**> FD A --> C implies the MVD B ->-> A | C, it is only due to the fact that B
**> appears in the relation schema that the MVD holds.
*

Correction: A --> C does not imply B ->->A | C; B --> C does. But the MVD B ->-> A | C and the FD A --> C cannot both hold unless at least one of the FDs B --> A or B --> C also holds.

> Therefore, in order to maintain the functional relationship from A to C,

*> it is necessary to add the IND {A,B}[B] in {B,C}[B].
**>
*

Received on Mon Aug 27 2007 - 18:30:18 CEST