Data integration consists in providing a uniform view of a set of heterogeneous data sources. This allows users to define their queries without any knowledge on the heterogeneous sources. Data integration systems use the mediation architecture to provide integrated access to multiple data sources. A mediator is a software device that supports a mediation schema which captures user requirements, and a set of mappings between the mediation schema and the distributed sources. Mediation systems [20] can be classified according to the approach used to define the mappings between the data sources and the global schema [12,23]. The first approach is called global-asview (GAV) and requires that each object of the global schema be expressed as a view (i.e. a query) on the data sources [6]. In the other approach, called local-as-view (LAV), mediation mappings are defined in an opposite way; each object in a given source is defined as a view on the global schema [11]. Both approaches allow to transform user queries, defined over the mediation schema, into subqueries defined over data sources. The transformation process is called a rewriting process and is done differently, depending on the approach used.