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We start with a very brief description of the many-valued logic approach to the formal treatment of null values in data bases and show some problems that are encountered. In the second part of the paper we present our approach, based on the Denotational Semantics Theory. An informal introduction to the theory is given and then the relational model is described in terms of the theory. Query evaluations are defined as continuous functions and several examples are presented. The formalization in the framework of Denotational Semantics allows for a better understanding of the semantic problems with null values. It also gives flexibility for alternatives in acceptable semantic interpretations. We conclude with a presentation of an algorithm for the evaluation of simple queries. This algorithm uses symbolic manipulations and it is more economical than the strict application of the definition of query evaluations.