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TopDuring last years several conjunctive query and meta-query languages were introduced and implemented on top of existing OWL reasoners. Most of the efforts have been spent on conjunctive ABox queries (i.e. queries that retrieve only ABox individuals). Authors of (Horrocks & Tessaris, 2000) propose methods for conjunctive ABox query answering in the ALC language (Baader, Calvanese, McGuinness, Nardi, & Patel-Schneider, 2003) that transforms a conjunctive query answering problem to instance checking or instance retrieval problem. Although for ALC the proposed technique works fine, the authors noticed that its generalization is problematic for expressive description logics backing OWL or OWL 2 and is possible only for queries that do not contain cyclic structures of undistinguished variables (see section Preliminaries for more details). Evaluation and optimization of conjunctive ABox queries in expressive description logics are discussed also in Sirin and Parsia (2006), Ortiz, Calvanese, and Eiter (2006) and in Dolby et al. (2008).
Conjunctive ABox queries allow retrieving individuals and literals from an OWL ontology, thus being similar to SQL query language for relational databases. However, OWL ontologies contain also significant amount of knowledge in TBox and RBox to model taxonomies and complex characteristics of classes (disjointness, equivalence, etc.) or properties (transitivity, functionality). To address this issue, query languages SPARQL-DL, SQWRL (O’Connor & Das, 2009) and OWL-SAIQL (Kubias, Schenk, Staab, & Pan, 2007) appeared during last years that allow evaluating mixed ABox, TBox and RBox queries to retrieve individuals, classes, and properties.