@prefix foaf: . @prefix ns1: . @prefix ns2: . ns1:claudio-gutierrez foaf:made ns2:iswc-2018-research-157 . ns1:aidan-hogan foaf:made ns2:iswc-2018-research-157 . ns1:daniel-hernandez foaf:made ns2:iswc-2018-research-157 . @prefix ns3: . @prefix ns4: . ns4:proceedings ns3:hasPart ns2:iswc-2018-research-157 . @prefix rdf: . ns2:iswc-2018-research-157 rdf:type ns3:InProceedings . @prefix rdfs: . ns2:iswc-2018-research-157 rdfs:label "Revisiting SPARQL's Semantics for Blank Nodes" . @prefix dc: . ns2:iswc-2018-research-157 dc:creator ns1:aidan-hogan , ns1:daniel-hernandez , ns1:claudio-gutierrez ; ns3:abstract "Blank nodes in RDF graphs can be used to represent existential values known to exist but whose identity remains unknown. A prominent example of such usage can be found in the Wikidata dataset where, e.g., the author of Beowulf is given as a blank node. However, while SPARQL considers blank nodes in a query as existentials, it treats blank nodes in data more like constants. Running SPARQL queries over datasets with unknown values thus may lead to uncertain results, which may make the SPARQL semantics unsuitable for datasets with existential blank nodes. We thus explore the feasibility of an alternative SPARQL semantics based on certain answers. In order to estimate the performance costs that would be associated with such a change in semantics for current implementations, we adapt and evaluate approximation techniques proposed in a relational database setting for a core fragment of SPARQL. To further understand the impact that such a change in semantics may have on query solutions, we analyse how such a change would affect the results of user queries over Wikidata." . @prefix ns8: . ns2:iswc-2018-research-157 ns3:hasAuthorList ns8:iswc-2018-research-157 ; ns3:isPartOf ns4:proceedings . @prefix xsd: . ns2:iswc-2018-research-157 ns3:keyword "query"^^xsd:string , "blank nodes"^^xsd:string , "semantics"^^xsd:string , "sparql"^^xsd:string ; ns3:title "Revisiting SPARQL's Semantics for Blank Nodes" . @prefix ns10: . ns10:iswc2018-author-aidan-hogan-iswc-2018-research-157 ns3:withDocument ns2:iswc-2018-research-157 . ns10:iswc2018-author-claudio-gutierrez-iswc-2018-research-157 ns3:withDocument ns2:iswc-2018-research-157 . ns10:iswc2018-author-daniel-hernandez-iswc-2018-research-157 ns3:withDocument ns2:iswc-2018-research-157 .