Question

WHY 3 NF IS VERY COMMON?

Why many designs are refined till 3NF only rather than proceeding to BCNF?

 

Any link to clarify this concept?

Answer 1

The reason for the 3NF being most widely used is :

A) Less no of joins :

As BCNF decompositions will lead to more number of tables possibly , so more number of joins may need to be required  for processing a query and hence consequently the query processing time will increase in such cases..

B) (The more important reason) Dependency preservation not satisfied always: 

The most important reason where 3NF has an edge over BCNF is while converting 3NF into BCNF it may be possible that all dependencies which are in the original relation are not satisfied  in the subrelation sometimes while trying to preserve the BCNF property..So 

BCNF does not guarantee dependency preservation while there exists a dependency preserving decomposition which is in 3NF always..

Thus 3NF is more in actual practice as compared to BCNF although BCNF is 0 % redundant over FDs.. 

Answer 2

Most 3NF tables are free of update, insertion, and deletion anomalies. Certain types of 3NF tables, rarely met with in practice, are affected by such anomalies; these are tables which either fall short of Boyce–Codd normal form(BCNF) or, if they meet BCNF, fall short of the higher normal forms 4NF or 5NF.