Abstract
RDF generally follows the open-world assumption: information is incomplete by default. Consequently, SPARQL queries cannot retrieve with certainty complete answers, and even worse, when they involve negation, it is unclear whether they produce sound answers. Nevertheless, there is hope to lift this limitation. On many specific topics (e.g., children of Trump, Apollo 11 crew, EU founders), RDF data sources contain complete information, a fact that can be made explicit through completeness statements. In this work, we leverage completeness statements over RDF data sources to provide guarantees of completeness and soundness for conjunctive SPARQL queries. We develop a technique to check whether query completeness can be guaranteed by taking into account also the specifics of the queried graph, and analyze the complexity of such checking. For queries with negation, we approach the problem of query soundness checking, and distinguish between answer soundness (i.e., is an answer of a query sound?) and pattern soundness (i.e., is a query as a whole sound?). We provide a formalization and characterize the soundness problem via a reduction to the completeness problem. We further develop heuristic techniques for completeness checking, and conduct experimental evaluations based on Wikidata, a prominent, real-world knowledge base, to demonstrate the feasibility of our approach.