| Abstract |
The ability of a query language to support mechanisms for defining views over a data set has been recognized as an added-value functionality to the expressive power of a query language in traditional database systems, such as relational and object-oriented systems, but also in modern database applications based on the semistructured and XML data models. The view definition mechanism enables the organization of (part of) a database into logic units, thus structuring (portions of) the database in a way suitable to the needs of specific applications. In the context of the Semantic Web, information consumers of the same information set, pose different requirements to the way they view information, hence necessitating the existence of mechanisms, such as views, to provide tailored access to the information sources. This need becomes more imperative, if we take into account the diversity of the representations used by information consumers and providers for formalizing, even the same, domain of discourse. Recognizing the need of a view definition mechanism for the Semantic Web, the present thesis specifies a view definition language for supporting the functionality needed by semantic web applications. Based on the data model of the Resource Description Framework (RDF/S) and by taking advantage of the expressiveness of the RQL query language for RDF/S graphs, the proposed view definition language, called RVL, incorporates in a uniform way the functionality needed and the peculiarities of the underlying data model. Being the first integrated effort for a view definition language specification, RVL exploits the RQL type system and the abstraction levels of an RDF/S graph to specify two operators, which are able to support all the necessary functionality. This minimality constitutes the most important advantage of RVL.
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RVL: Μια γλώσσα ορισμού όψεων για το πρότυπο RDF/S
