Database systems have evolved from simple record-orientednavigational database systems, hierarchical and network systems,into set-oriented systems that gave way to relational databasesystems. The relational database systems are now evolving intoobject-relational and multimedia database systems. During the lastyears, database systems have been used as platforms for managingdata for conventional transaction-oriented business applications. Asorganizations and companies have become more sophisticated,pressure to provide data integration across different, heterogeneousdata sources has mounted. The problem how to provide organization-wide uniform access to heterogeneous and autonomous external datasources (file systems, databases, Web pages, etc.) can be addressed,genrally, in three different ways: federated databases, datawarehousing systems, and mediated systems. Database systems haveevolved from simple record-oriented navigational database systems,
