A survey on SPARQL query relaxation under the lens of RDF reification

Query relaxation has been proposed to cope with the problem of queries that produce none or insufficient answers. The goal is to modify these queries to be able to produce alternative results close to those expected in the original query. Existing approaches querying RDF datasets generally relax the SPARQL query constraints based on logical relaxations through RDFS entailment and RDFS ontologies. Techniques also exist that use the similarity of instances based on resource descriptions. These relaxation approaches defined for SPARQL queries over RDF triples have proved their efficiency. Nevertheless, significant challenges arise for query relaxation techniques in the presence of statement-level annotations, i.e., RDF reification. In this survey, we overview SPARQL query relaxation works with a particular focus on issues and challenges posed by representative RDF reification models, namely, standard reification, named graphs, n-ary relations, singleton properties, and RDF-Star.