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.