Final answer:
Normalizing to 2NF is not always sufficient because it does not eliminate all redundancy, does not always preserve all dependencies, may not help in identifying all key attributes correctly, and does not minimize all dependencies as BCNF does.
Step-by-step explanation:
The question asks why normalizing database tables to the Second Normal Form (2NF) is usually not sufficient and lists potential reasons, suggesting that it's often necessary to normalize further to the Boyce-Codd Normal Form (BCNF).
- Redundancy elimination: While 2NF does address some redundancy by removing partial dependencies, BCNF goes further by eliminating all redundancy caused by functional dependencies, thus ensuring that every determinant is a candidate key.
- Dependency preservation: In 2NF, some dependencies may not be fully preserved if they do not pertain to a composite key, whereas BCNF ensures all non-trivial dependencies are preserved.
- Key attribute identification: Moving to BCNF helps in clearly identifying candidate keys as it involves a stricter set of rules compared to 2NF.
- Minimization of partial dependencies: 2NF eliminates partial dependencies, but BCNF further minimizes dependencies by ensuring that all non-key attributes are fully functionally dependent on the primary key.
To summarize, even though 2NF helps in reducing some forms of data redundancy and helps in identifying some key attributes, BCNF provides a more robust structure by ensuring that every non-trivial functional dependency is on a superkey, and thus no redundancy due to functional dependencies remains.