Ontology Based Query Answering with Existential Rules

Ontology-Based Query Answering (OBQA) is currently a problem that receives a lot of attention both from knowledge representation and databases communities. The aim is to answer queries that are at least as expressive as conjunctive queries while taking an ontology into account. This is important in order to improve the quality of query answering and interoperability between different sources of data. The mainstream formalism to deal with ontologies is description logics (DLs) ([Baader et al., 2007]). While historical DLs are very expressive, most of the OBQA research focus on recently introduced lightweight DLs (EL [Baader, 2003] and DL-Lite [Calvanese et al., 2007]). Real-world ontologies expressed in these DLs already exist, such as the medical ontology SNOMED-CT (based on EL). During my Ph.D, I am considering an alternative, rulebased formalism. I use existential rules [Baget et al., 2011b] (rules for short, also known as Tuple Generating Dependencies [Abiteboul et al., 1994], Datalog+/[Calı̀ et al., 2009]), which despite their simple syntactic form are very expressive. Moreover, they allow for a smooth integration to database systems since, in contrast to DLs, they allow for any predicate arity and support variable cycles. Indeed, their form is: