# Re: BCNF

From: Brian Selzer <brian_at_selzer-software.com>
Date: Sat, 2 Aug 2008 04:10:39 -0400
Message-ID: <jEUkk.17965\$89.15424_at_nlpi069.nbdc.sbc.com>

> Hi all,
>
> BCNF
>
> the following is the definition is the definition of BCNF , which i
> saw in a schaum series book
>
> 1) The relation is 1 N.F
>
> 2) for every functional dependency of the form X -> A , we have
> either A C X or X is a super key of r. in other words,
> every functional dependency is either a trivial dependency or in
> the case that the functional dependency is not trivial then X must
> be a super key.
>
> now my questions are as follows
>
> 1)
>
> we know that 2-ND normal form is all about separating partial
> dependencies and full dependencies.third normal form is all about
> removing transitive dependencies, in these lines can any one give
> simple/ easy to understand method/explanation for converting a
> relation in 3rd normal form to BCNF
>

A relation schema is in 3NF iff for every functional dependency the determinant is a superkey or the dependent is prime; a relation schema is in BCNF iff every determinant is a superkey. A schema that is in 3NF but not in BCNF will have one or more determinants that are not superkeys. Find them and eliminate them.

>
> 2) how correct is the following definition of transitive
> dependencies
>
>
> transitive dependencies
>
> assume that A,B, and C are the set of attributes of a relation(R).
> further assume that the following
> functional dependencies are satisfied simultaneously : A -> B , B -/-
>> A, B -> C , and C -/-> A and A -> C
> observe that C -> B is neither prohibited nor required. if all these
> conditions are true, we will say that attribute C is transitively
> dependent on attribute on A
>

It is not correct: what if B = C or C is a subset of B? Received on Sat Aug 02 2008 - 10:10:39 CEST

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