CoBase: A Cooperative Database System

This chapter proposes the use of the type abstraction hierarchy (TAH) as a framework for deriving cooperative query answers (CQA). The TAH integrates abstraction with the subsumption (is-a) and composition (part-of) semantics found in the type hierarchy. This framework provides a multi-level object representation that is an important aspect of cooperative query answering. Patterns, which specify one or more conditions on an object, are introduced as a small granularity structure with speciic semantic information. Cooperative query answering uses the TAH and patterns to provide query relaxation, generalization and specialization. Relaxation can be explicitly controlled by the user, implicitly performed by the system, or both. An explanation system is also included to present the relaxation path as well as a nearness measure between the approximate answer and the exact answer. The operations required for CQA may be added to any conventional query language. As an example, we present CSQL, a cooperative extension of SQL used in the CoBase project at UCLA. Experimental results reveal that the proposed type abstraction hierarchy and relaxation control provide an organized structure for representing concepts at diierent knowledge levels in various domains, and ooers a systematic and eecient method for cooperative query answering.