Nowadays there is an explosion of data repositories available in the Global Information System. In this context, users meet with the problems related to the heterogeneity and distribution of the data repositories. Access to the data may be facilitated by providing semantic views over the repositories that hide all the technical and organizative details associated with them. In this paper it is presented the definition of the mapping relation that permits one to relate data elements of the semantic views and those of the underlying data repositories. We define the syntax and the semantics of the mapping. From the syntactic point of view, mapping expressions are tuples which main components are relational algebra expressions. From the semantic point of view, the notions of well formed mapping relation and the extension defined by that mapping are introduced. Moreover, we demonstrate the completeness property of the mapping relation in the sense that it allows for answers to all queries formulated over the semantic view using the data stored in the data repositories. Copyright c fl1999 Elsevier Science Ltd Key words: Terminological Systems, Mapping Information, Global Information Systems, Data Repositories 1.

Abstract. Query reformulation techniques based on semantic knowledge have been used in two ways in database management systems, namely for query optimization and for data integration. For the former approach, the main goal of query reformulation is to rewrite a user query into another one that uses less time and/or less resources during the execution. The main goal of the latter approach is to translate a user query into a set of queries which fit best the structure of the distributed sources. When using those query optimization strategies the transformed queries are equivalent to the submitted ones, i.e. they provide the same answer set, whereas when using data integration strategies a loss of information is expected. This paper presents a new approach of query reformulation using ontology semantics for query processing within a single relational database system. Here, the aim of the query reformulation is to extend the result of a given query in a semantically meaningful way. In fact, our approach shows how an ontology can effectively be exploited to rewrite a user query into another one such that the new query provides additional meaningful results that satisfy the intention of the user. Based on practical examples and their usefulness we develop a set of reformulation rules. In addition, we prove that the results of the reformulations are semantically correct by using a logical model. 1

This thesis describes an ontology-based query-centric federated solution to information extraction from heterogeneous, autonomous, distributed information sources.