CG-SQL: a front-end language for conceptual graph knowledge bases

Conceptual Graph (CG) knowledge bases can be queried only by mean of goals. We show that some conventional operators (projection, selection, union, cartesian product) can not be expressed unless a set of rules is added to the knowledge base prior to the inference, which is highly inconvenient. Therefore, we propose an algebra and a declarative SQL-like language for the CG model, which permits the expression of complex queries with the above four operators, leaving the knowledge base unchanged. We prove that CG-SQL is complete with respect to safe sets of rules and that queries are decidable if no unsafe rules are present in the knowledge base.