Until this book, the available literature on fuzzy sets has been, at best, scattered throughout industrial and university libraries. Encapsulated here is a sound discussion of the basic theoretical and practical aspects involved in fuzzy database systems. With a practical, hands-on, applications-oriented approach, it develops computer models for applications to decision-making processes, introducing the basic notion of relative grades via the fuzzy set theoretic approach. Also covers fuzzy relational databases and their calculus, and the fuzzy relational (structured) query language (FSQL). The last sections present methods for treating the incomplete information in fuzzy PROLOG database (FPDB) systems. Several examples of knowledge representation, expert systems, fuzzy control, and fuzzy clustering and information retrieval illustrate the theory. An extended sample database is used throughout the book.