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Relational data exchange deals with translating a relational database instance over some source schema into a relational database instance over some target schema, according to a schema mapping that specifies the relationship between the source data and the target data. Various semantics for answering queries against the target schema exist, each of them suitable for a certain class of queries, and with respect to certain schema mappings. However, for each of these semantics, there are examples that show that it leads to counter-intuitive answers, or that it does not respect logical equivalence of schema mappings.
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