Abstract Information systems have to deal with an increasing amount of data that is
heterogeneous, unstructured, or incomplete. In order to align and complete data, systems …

This chapter accompanies the foundational lecture on Description Logics (DLs) at the 7th
Reasoning Web Summer School in Galway, Ireland, 2011. It introduces basic notions and …

Recent years have seen an increasing interest in ontology-mediated query answering, in
which the semantic knowledge provided by an ontology is exploited when querying data …

Answering conjunctive queries (CQs) over a set of facts extended with existential rules is a
prominent problem in knowledge representation and databases. This problem can be …

The high computational complexity of the expressive Description Logics (DLs) that underlie
the OWL standard has motivated the study of their Horn fragments, which are usually …

Ontology-based data access (OBDA) generalizes query answering in relational databases. It
allows to query a database by using the language of an ontology, abstracting from the actual …

Logics (DLs) play a central role as formalisms for representing ontologies and reasoning
about them. This lecture introduces the basics of DLs. We discuss the knowledge modeling …

Answering conjunctive queries over ontology-enriched datasets is a core reasoning task for
many applications. Query answering is, however, computationally very expensive, which …

We perform an in-depth complexity analysis of query answering under guarded-based
classes of disjunctive tuple-generating dependencies (DTGDs), focusing on (unions of) …

Ontology-based data access (OBDA) generalizes query answering in databases towards
deduction since (i) the fact base is not assumed to contain complete knowledge (ie, there is …