Approximate Query Answering over Inconsistent Knowledge Bases Sergio Greco, Cristian Molinaro, Irina Trubitsyna DIMES, University of Calabria {greco,cmolinaro,trubitsyna}@dimes.unical.it Abstract. Consistent query answering is a principled approach for querying inconsistent knowledge bases. It relies on the central notion of repair, that is, a maximal consistent subset of the facts in the knowledge base. One drawback of this approach is that entire facts are deleted to resolve inconsistency, even if they may still contain useful “reliable” information. To overcome this limitation, we propose an inconsistency-tolerant semantics for query answering based on a new notion of repair, allowing values within facts to be updated for restoring consistency. This more fine-grained repair primitive allows us to preserve more information in the knowledge base. We also introduce the notion of a universal repair, which is a compact representation of all repairs and can be computed in polynomial time. Then, we show that consistent query answering in our framework is intractable (coNP-complete). In light of this result, we develop a polynomial time approximation algorithm for computing a sound (but possibly incomplete) set of consistent query answers. 1 Introduction Reasoning in the presence of inconsistent information is a problem that has attracted much interest in the last decades. Many inconsistency-tolerant semantics for query answering have been proposed, and most of them rely on the notions of consistent query answer and repair. A consistent answer to a query is a query answer that is entailed by every repair, where a repair is a “maximal” consistent subset of the facts of the knowledge base. Different maximality criteria have been investigated, but all the resulting notions of repair share the same drawback: a fact is either kept or deleted altogether, and deleting entire facts can cause loss of “reliable” information, as illustrated below. Example 1. Consider the knowledge base (D,Σ), where D contains the following facts: works john cs nyc john math rome mary math sidney and Σ is an ontology consisting of the following equality-generating dependency σ: works(E 1 , D, C 1 ) ∧ works(E 2 , D, C 2 ) → C 1 = C 2 . SEBD 2018, June 24-27, 2018, Castellaneta Marina, Italy. Copyright held by the author(s).