{"type":"ScholarlyArticle","title":"The Bag Semantics of Ontology-Based Data Access","authors":[{"name":"Charalampos Nikolaou","slug":"charalampos-nikolaou","given":"Charalampos","family":"Nikolaou","affiliation":"Department of Computer Science , University of Oxford , UK"},{"name":"Egor V. Kostylev","slug":"egor-v-kostylev","given":"Egor V.","family":"Kostylev","affiliation":"Department of Computer Science , University of Oxford , UK"},{"name":"George Konstantinidis","slug":"george-konstantinidis","given":"George","family":"Konstantinidis","affiliation":"Department of Computer Science , University of Oxford , UK"},{"name":"Mark Kaminski","slug":"mark-kaminski","given":"Mark","family":"Kaminski","affiliation":"Department of Computer Science , University of Oxford , UK"},{"name":"Bernardo Cuenca Grau","slug":"bernardo-cuenca-grau","given":"Bernardo Cuenca","family":"Grau","affiliation":"Department of Computer Science , University of Oxford , UK"},{"name":"Ian Horrocks","slug":"ian-horrocks","given":"Ian","family":"Horrocks","affiliation":"Department of Computer Science , University of Oxford , UK"}],"abstract":"Ontology-based data access (OBDA) is a popular approach for integrating and querying multiple data sources by means of a shared ontology. The ontology is linked to the sources using mappings, which assign views over the data to ontology predicates. Motivated by the need for OBDA systems supporting database-style aggregate queries, we propose a bag semantics for OBDA, where duplicate tuples in the views defined by the mappings are retained, as is the case in standard databases. We show that bag semantics makes conjunctive query answering in OBDA coNP-hard in data complexity. To regain tractability, we consider a rather general class of queries and show its rewritability to a generalisation of the relational calculus to bags.","doi":"10.24963/ijcai.2017/170","url":"https://ijcai.org/proceedings/2017/170","pdf":"https://ijcai.org/proceedings/2017/0170.pdf","markdown":"https://ijcai.org/proceedings/2017/170.md","bibtex":"https://ijcai.org/proceedings/2017/bibtex/170","volume":{"slug":"2017","title":"Proceedings of the Twenty-Sixth International Joint Conference on Artificial Intelligence","short_title":"IJCAI 2017","url":"https://ijcai.org/proceedings/2017/","year":2017,"location":"Melbourne, Australia","dates":"19-25 August 2017","editors":"Carles Sierra","isbn":"978-0-9992411-0-3","publisher":"International Joint Conferences on Artificial Intelligence Organization","doi_prefix":"10.24963/ijcai.2017"},"section":"Knowledge Representation, Reasoning, and Logic","track":"Main track","number":170,"pages":{"first":1224,"last":1230},"keywords":["Knowledge Representation, Reasoning, and Logic: Knowledge Representation Languages","Knowledge Representation, Reasoning, and Logic: Description Logics and Ontologies"],"videos":[],"references":[{"position":1,"raw":"[Afrati and Kolaitis, 2008] Foto N. 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