Answering Conjunctive Queries with Inequalities in <em>DL-Lite</em><sub>ℛ</sub>

  • Gianluca Cima Sapienza University of Rome
  • Maurizio Lenzerini Sapienza University of Rome
  • Antonella Poggi Sapienza University of Rome


In the context of the Description Logic DL-Lite, i.e., DL-Lite without UNA and with inequality axioms, we address the problem of adding to unions of conjunctive queries (UCQs) one of the simplest forms of negation, namely, inequality. It is well known that answering conjunctive queries with unrestricted inequalities over DL-Lite ontologies is in general undecidable. Therefore, we explore two strategies for recovering decidability, and, hopefully, tractability. Firstly, we weaken the ontology language, and consider the variant of DL-Lite corresponding to rdfs enriched with both inequality and disjointness axioms. Secondly, we weaken the query language, by preventing inequalities to be applied to existentially quantified variables, thus obtaining the class of queries named UCQ≠,bs. We prove that in the two cases, query answering is decidable, and we provide tight complexity bounds for the problem, both for data and combined complexity. Notably, the results show that answering UCQ≠,bs over DL-Lite ontologies is still in AC0 in data complexity.

AAAI Technical Track: Knowledge Representation and Reasoning