One of the objectives of the MeDoc system is the possibility of querying distributed heterogeneous provider systems. This paper describes how the MeDoc system deals with this heterogeneity. We take an approach of multiple layers. The users formulate their queries with respect to the global schema which is then transformed into the conceptual schema, then further into the physical schema, and finaly into the protocol of the provider system. This transformation process does not require equivalent schemata of the connected provider systems, a best-match approach is chosen instead, so as not to unnecessarily restrict the information sources that can be added to the system.
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