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Outer joins are extended relational algebra operations intended to deal with unknown information represented with null values. This work shows an approach to embed both null values and outer join operations in the deductive database system DES (Datalog Educational System), which uses Datalog as a query language. This system also supports SQL, where views and queries are compiled to Datalog programs. So, as SQL statements are ultimately solved by a Datalog engine, it became a need to integrate null-related operations into Datalog in order to support a wider set of SQL. Since DES implements a top-down-driven bottom-up stratified fixpoint computation based on tabling for solving Datalog queries, we show how to compute outer joins in such a context by means of source-to-source transformations applied to Datalog programs.