One of the most popular applications of Semantic Web technology is the integration of data from distributed locations over the Web. With wrappers, screen scrapers, and information extraction tools it is possible to access, merge, and reason over RDF data from various different, heterogeneous sources. SPARQL can be used to access RDF data in a declarative manner and as will be shown, it can also be used to virtually integrate distributed RDF graphs. In this contribution a mediator-wrapper based middleware is shown which virtually integrates distributed, heterogeneous data sources based on RDF and a slightly modified SPARQL algebra. Schema information available in RDF Schema and OWL vocabularies is used for query federation and optimization. Compared to traditional data integration systems, SemWIQ can fully exploit the semantics of available RDF data. Additionally, RDF has many interesting properties which provide advanced query capabilities based on OWL DL inference.