This paper addresses the problem of querying semantic data from heterogeneous web sources. On one hand, centralized knowledge graphs, such as RDF stores, can be accessed with flexibility and efficiency using SPARQL queries. On the other hand, distributed knowledge graphs, such as microdata, are not directly queryable, and are rather exploited by search engines. RDF stores and microdata provide complementary information: RDF stores typically offer higher-quality data, while microdata delivers fresher content. The research problem considered in this paper is the hybrid querying of a centralized RDF store and distributed microdata on the web. To this aim, we introduce Web-SPARQL, an extension of SPARQL with property functions that link the centralized entities to the distributed entities on the web.