Computer Science Workbench is a monograph series which will provide you with an in-depth working knowledge of current developments in computer technology. Every volume in this series will deal with a topic of importance in computer science and elaborate on how you yourself can build systems related to the main theme. You will be able to develop a variety of systems, including computer software tools, computer graphics, computer animation, database management systems, and computer-aided design and manufacturing systems. Computer Science Workbench represents an important new contribution in the field of practical computer technology. TOSIYASU L. KUNII Preface The evolution of database systems research is itself a story. Long after the emergence of systems derived from practical applications, Codd's relational data model has gradually occupied the theoretical domain of database systems and is moving into the realms of practical use. Certainly, the theoretical foundation makes database design, validation, and testing easier. However, Cod d's model allows only fiat tables to be handled, while most business and engineering data in practice are in nested table forms. Thus, a recent major obstacle in database systems development is the large gap between the theory and the practice.