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Main contribution of this paper is an investigation of expressive power of the database category DB. An object in this category is a database-instance (Set of n-ary relations). Morphisms are not functions but have complex tree structures based on a set of complex query computations. They express the semantics of view-based mappings between databases. The higher (Logical) level scheme mappings between databases, usually written in some high expressive logical language, may be functorially translated into this base "Computation" DB category. The behavioral point of view for databases is assumed, with behavioural equivalence of databases corresponding to isomorphism of objects in DB category.
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