In the database engineering realm, the merits of transformational approaches, that can produce in a systematic way correct, compilable and efficient database structures from abstract models, has long be recognized. Transformations that are proved to preserve the correctness of the source specifications have been proposed in virtually all the activities related to data structure engineering: schema normalization, logical design, schema integration, view derivation, schema equivalence, data conversion, reverse engineering, schema optimization, wrapper generation and others. This paper addresses both fundamental and practical aspects of database transformation techniques. The concept of transformation is developed, together with its properties of semantics-preservation (or reversibility). Major database engineering activities are redefined in terms of transformation techniques, and the impact on CASE technology is discussed. These principles are applied to database logical design and database reverse engineering. They are illustrated by the use of DB-MAIN, a programmable CASE environment that provides a large transformational toolkit.
