# Re: BCNF

Date: Fri, 01 Aug 2008 08:49:01 -0700

Message-ID: <1217605733.949337@bubbleator.drizzle.com>

aarklon_at_gmail.com wrote:

*> 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*

*> *

*> *

*> 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*

Personally I prefer the following definition:

Boyce Codd Normal Form (BCNF) is a further refinement of 3NF. In his later writings Codd refers to BCNF as 3NF. A row is in Boyce Codd normal form if, and only if, every determinant is a candidate key. Most entities in 3NF are already in BCNF.

BCNF covers very specific situations where 3NF misses inter-dependencies between non-key (but candidate key) attributes. Typically, any relation that is in 3NF is also in BCNF. However, a 3NF relation won't be in BCNF if (a) there are multiple candidate keys, (b) the keys are composed of multiple attributes, and (c) there are common attributes between the keys.

-- Daniel A. Morgan Oracle Ace Director & Instructor University of Washington damorgan_at_x.washington.edu (replace x with u to respond) Puget Sound Oracle Users Group www.psoug.orgReceived on Fri Aug 01 2008 - 10:49:01 CDT