In the recent years establishing interoperability and supporting data integration has become a major research challenge for the web of data. Uniform information access of heterogeneous sources is of major importance for Semantic Web applications and end users. We describe a methodology for SPARQL query mediation over federated OWL/ RDF knowledge bases. The query mediation exploits mappings betw...

Considerable effort has been made to improve the functionality and usability of SPARQL search engines. However, explaining missing items in the results of SPARQL queries or the so-called why-not questions remains in its infancy. Existing explanation models cannot be trivially extended to SPARQL queries because of the SPARQL-specific features in the data model and query operations. In this demon...

Recent studies reveal that publicly available SPARQL endpoints exhibit significant limitations in supporting real-world applications. In order for this querying infrastructure to reach its full potential, more flexible client-server architectures capable of deciding appropriate shipping plans are needed. Shipping plans indicate how the execution of query operators is distributed between the cli...

In the last years, the query language SPARQL has evolved into the widely accepted standard for querying RDF. Since many Semantic Web applications make use of data whose storage and management is distributed, distributed SPARQL query processing becomes necessary. In the relation and object-oriented database community the efficiency gain by cost-based, adaptive optimizers for distributed querying...

The Semantic Web is the initiative of the World Wide Web Consortium (W3C) to make information on the Web readable not only by humans but also by machines. RDF is the data model for Semantic Web data, and SPARQL is the query language for this data model. RDF and SPARQL are the output of long standardizations processes led by the W3C. These processes, usually in charge of developers, are not alwa...

Due to the decentralised and autonomous architecture of the Web of Data, data replication and local deployment of SPARQL endpoints is inevitable. Nowadays, it is common to have multiple copies of the same dataset accessible by various SPARQL endpoints, thus leading to the problem of selecting optimal data source for a user query based on data properties and requirements of the user or the appli...

SPARQL is a standard query language for retrieving and manipulating RDF data. However, the majority of SPARQL implementations require the data to be available in advance, i.e., to exist in main memory or in a RDF repository (accessible through a SPARQL endpoint). Nonetheless, Linked Data exists in the Web in various forms; even an HTML Web page can contain RDF data through RDFa, or RDF data may...

Linked data is increasingly available through SPARQL endpoints, but exploration and question answering by regular Web users largely remain an open challenge. Users have to choose between the expressivity of formal languages such as SPARQL, and the usability of tools based on navigation and visualization. In a previous work, we have proposed Query-based Faceted Search (QFS) as a way to reconcile...