Re: Sixth normal form

"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. 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 Sat Aug 25 2007 - 03:15:02 CEST