Query Answering under Expressive Entity-Relationship Schemata

We address the problem of answering conjunctive queries under constraints representing schemata expressed in an extended version of the Entity-Relationship model. This extended model, called ER+ model, comprises is-a constraints among entities and relationships, plus functional and mandatory participation constraints. In particular, it allows arbitrary permutations of the roles in is-a among relationships. A key notion that ensures high tractability in ER+ schemata is separability, i.e., the absence of interaction between the functional participation constraints and the other constructs of ER+. We provide a precise syntactic characterization of separable ER+ schemata, called ER schemata, by means of a necessary and sufficient condition. We present a complete complexity analysis of the conjunctive query answering problem under ER schemata. We show that the addition of so-called negative constraints does not increase the complexity of query answering. With such constraints, our model properly generalizes the most widely-adopted tractable ontology languages.