  The World Wide Web currently evolves into a Web of Linked Data where content
providers publish and link data as they have done with hypertext for the last
20 years. While the declarative query language SPARQL is the de facto for
querying a-priory defined sets of data from the Web, no language exists for
querying the Web of Linked Data itself. However, it seems natural to ask
whether SPARQL is also suitable for such a purpose.
  In this paper we formally investigate the applicability of SPARQL as a query
language for Linked Data on the Web. In particular, we study two query models:
1) a full-Web semantics where the scope of a query is the complete set of
Linked Data on the Web and 2) a family of reachability-based semantics which
restrict the scope to data that is reachable by traversing certain data links.
For both models we discuss properties such as monotonicity and computability as
well as the implications of querying a Web that is infinitely large due to data
generating servers.
