Abstract-Band - Fakultät für Informatik, TU Wien - Technische ...

more recently, query answering. Besides explaining the presence of a tuple in
a query answer (explanation of positive answer), it is important to explain also
why a given tuple is missing (explanation of negative answer). We first
address the problem of explaining positive query answers by providing an
algorithmic solution to the procedure already outlined in the literature.
Additionally, we employ the new algorithm to solve the explanation problem
for inconsistency in DL-Lite ontologies. In solving the problem of explaining
negative answers to (U)CQ over ontologies, we adopt abductive reasoning,
that is, we look for additions to the ABox that force a given tuple to be in the
result. As reasoning tasks, we consider existence and recognition of an
explanation, and relevance and necessity of a certain assertion for an
explanation. We characterize the computational complexity of these problems
for subset minimal and cardinality minimal solutions.
Trung Kien Tran
Query Answering in the Description Logic Horn-SHIQ
Studium: Masterstudium DDP Computational Logic (Erasmus-Mundus)
Betreuer: O.Univ.Prof. Dr. Thomas Eiter
Ontologies can be used to formally represent knowledge of an application
domain. This enables machines to make use of the knowledge, to discover
new knowledge, or to exchange knowledge with others. In the context of
database systems, an ontology can be seen as the conceptual view over data
repositories, and data can be accessed by query answering services. To build
such ontologies, we need ontology languages. Among formalisms for
ontology languages, Description Logics (DLs) are the most prominent ones.
DLs underlie the foundation for the current Web Ontology Language (OWL)
standard recommended by World Wide Web Consortium (W3C). DLs are a
family of knowledge representation languages with different expressive
capabilities and computational complexities. It seems that only lightweight
languages i.e., language with low complexity, are suitable for applications
dealing with a large amount of data. The first family of lightweight DLs is DL-
Lite, specially designed to capture data modeling formalism while keeping
very low complexity. The second collection of lightweight languages is the EL
family. Both of these families are subsumed by the Description Logic Horn-
SHIQ. Horn-SHIQ is the Horn fragment of the expressive Description Logic
SHIQ underlying the OWL-Lite standard. There exist practical algorithms for
query answering in EL and DL-Lite but there is no such algorithms for Horn-
SHIQ. In this thesis, we investigate this issue and propose a query rewritingbased
approach for answering conjunctive queries over ontologies in Horn-
SHIQ. Apart from providing a sound and complete algorithm, we have
implemented a prototype system and performed experiments on well-known
ontologies.
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