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

Conceptual Graph (CG) knowledge bases can be queried only by means of goals . We show that some conventional operators (projection, selection, union, c artesian product) cannot be expressed unless a set of rules is added to the knowledge base prior to the inference, which is highly inconvenient. There fore, 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 co mplete with respect to safe sets of rules and that queries are decidable if no unsafe rules an present in the knowledge base. (C) 1999 Elsevier Scienc e B.V. All rights reserved.