Querying the Web Reconsidered: Design Principles for Versatile Web Query Languages

A decade of experience with research proposals as well as standardized query languages for the conventional Web and the recent emergence of query languages for the Semantic Web call for a reconsideration of design principles for Web and Semantic Web query languages. This article first argues that a new generation of versatile Web query languages is needed for solving the challenges posed by the changing Web: We call versatile those query languages able to cope with both Web and Semantic Web data expressed in any (Web or Semantic Web) markup language. This article further suggests that (well-known) referential transparency and (novel) answer-closedness are essential features of versatile query languages. Indeed, they allow queries to be considered like forms and answers like form-fillings in the spirit of the “query-by-example” paradigm. This article finally suggests that the decentralized and heterogeneous nature of the Web requires incomplete data specifications (or “incomplete queries”) and incomplete data selections (or “incomplete answers”): the form-like query can be specified without precise knowledge of the queried data and answers can be restricted to contain only an excerpt of the queried data.