The Datalog Educational System (DES) is a free, open-source, multiplatform, portable, Prolog-based implementation of a deductive database system. DES includes Datalog, SQL, and Relational Algebra as query languages. It features full recursive evaluation with tabling, full-fledged arithmetic, stratified negation, duplicates, integrity constraints, ODBC connections to external relational database management systems, Datalog and SQL declarative debuggers and tracers, a textual API for external applications for any host language (Java, C++, ...), extended Datalog (outer joins, aggregates, hypothetical queries, ordering, top-N answers, ...), extended SQL language (hypothetical SQL queries, division relational operation, ...), and test case generation for SQL views, among others. See also What’s New, Key Features, and Read More | 
