Introduction Database system technology has reached a stage now in which there is a proliferation of independent systems storing and manipulating enormous amount of data. Unfortunately, these systems typically have their own data models, communication processing protocols, query processing systems, concurrency control protocols, consistency management, and other similar aspects of database systems. There is also an increasing need for Interoperability among these systems. Though considerable amount of research has been done in the area of database interoperability, most of it has resulted in solutions that are ad-hoc and procedural. We have developed a declarative environment in which multiple heterogeneous databases interoperate by sharing, interpreting, and manipulating information, in a uniform way. An important criterion for Interacting with multiple databases is the ability to query them in a manner independent of the discr