With thousands of RDF data sources today available on the Web, covering disparate and possibly overlapping knowledge domains, the problem of providing high-level descriptions (in the form of metadata) of their content becomes crucial.

We studied the problem of describing data sources in terms of their completeness. Indeed, existing data sources can be described with completeness statements expressed in RDF. Completeness statements are useful for query answering, that is, providing information whether the answer to a query is complete or not. The CORNER system is an implementation of a reasoner to assess query completeness from completeness statements about a data source.