Schema Transformations and Dependency Preservation

For developing good quality information systems, the need of developing good quality conceptual models cannot be over emphasized. To improve the quality of a conceptual model, schema transformation rules have been proposed in our previous research. For applications developed using relational database management systems, conceptual models are translated into relational schema and the quality of a relational schema is determined in terms of the normal form satisfied by each relation in the database schema. Though various normal forms have been proposed in the literature, database designers, in practice, usually normalize a relational database schema up to third normal form (3NF) or Boyce-Codd Normal Form (BCNF). Deciding between 3NF and BCNF has always been an important design issue because a BCNF decomposition of a relation schema may lose dependency preservation. In this paper, we identify properties of the relations which are in 3NF and not in BCNF. Some of these properties have been informally stated in text books or in the literature without any formal proofs. We state these properties clearly and also present their formal proofs. This research facilitates in devising schema transformation rules that can further improve the quality of a conceptual model.