The Return of the Entity-Relationship Model: Ontological Query Answering

Abstract

The Entity-Relationship (ER) model is a fundamental tool for database design, recently extended and employed in knowledge representation and reasoning due to its expressiveness and comprehensibility. We present an extension of the ER model, called ER ⁺, which is particularly suitable for ontology modeling, as well as being flexible and comprehensible. Our model comprises is-a constraints among entities and relationships, plus functional and mandatory participation constraints. In particular, it allows for arbitrary permutations of the roles in is-a among relationships. We argue that ER-based languages can be profitably used in ontology-enhanced database systems, where queries are evaluated against the union of a database instance and an ontology, which constitute a logical theory. In such systems, the instance has usually large size, therefore ensuring tractable complexity of query answering w.r.t. the instance size is crucial. A key notion that ensures tractability in ER ⁺schemata is separability, i.e., the absence of interaction between the functional participation constraints and the other constructs. We provide a precise syntactic characterization of separable ER ⁺schemata by means of a necessary and sufficient condition. We present a complete complexity analysis of the conjunctive query answering problem under separable ER ⁺schemata, and also under several sublanguages of ER ⁺. We show that the addition of so-called negative constraintsdoes not increase the complexity of query answering. With such constraints, our model properly generalizes the most widely-adopted tractable ontology languages, including those in the well-known DL-Litefamily.