A Survey on SPARQL Query Relaxation under the Lens of RDF Reification

SPARQL query relaxation has been used 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 generally relax the 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., reification. In this survey, we overview query relaxation works with a particular focus on issues and challenges posed by representative reification models, namely, standard reification, named graphs, n-ary relations, singleton properties, and RDF-Star.