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Data Dependencies in Codd's Relational Model with Similarities

This chapter deals with data dependencies in Codd’s relational model of data. In particular, we deal with fuzzy logic extensions of the relational model which consist in adding similarity relations to domains and consider functional dependencies in these extensions. We present a particular extension and functional dependencies in this extension which follows the principles of fuzzy logic in narrow sense. We present selected features and results regarding this extension. Then, we use this extension as a reference model and compare it to several other extensions proposed in the literature. We argue that following the principles of fuzzy logic in narrow sense the same way as following the principles of classical logic in case of ordinary Codd’s relational model helps achieve transparency, versatility, conceptual clarity, and theoretical and computational tractability of the extension. We outline several topics for future research.