Why is this important?

• A majority of dynamic Web content is backed by relational databases (RDB), and so are many enterprise systems.
• The emerging Web of Data, based on RDF, enables data sharing and integration on a Web scale.
• In late 2007, W3C organized a RDB2RDF workshop and identified the need to standardise the existing RDB2RDF mapping approaches.

Where can this be used?

• Health Care and Life Sciences
• eCommerce
• Enterprise data integration
• Web Content Management Systems
• ... see the Use Cases and Requirements for Mapping Relational Databases to RDF document for more details.

How does this work? (I)

How does this work? (II)

• Input: RDB and an (optional) mapping
• Output: RDF graph (virtual or materialized):
  1. query access: agent issues a SPARQL query against an endpoint
  2. entity-level access: agent performs an HTTP GET on a URI exposed by the system
  3. dump access: agent performs an HTTP GET on dump of the entire RDF graph, for example in Extract, Transform, and Load (ETL) process

Mappings overview

• If you don't specify a mapping explicitly, RDB2RDF defaults to a direct mapping
• ... however, you can define your own mapping using the RDB2RDF Mapping Language (R2RML)

Direct Mapping

RDB2RDF Mapping Language (R2RML)

R2RML overview

R2RML example

RDB (DEPT table)

Schema

Name	Datatype	Key Constraints
deptno	INTEGER	UNIQUE
dname	VARCHAR(30)
loc	VARCHAR(100)

Data

deptno	dname	loc
10	APPSERVER	NEW YORK

R2RML Mapping

<#ExampleTriplesMap>
 a rr:TriplesMapClass;
 rr:SQLquery "SELECT ('Department' || deptno) 
              AS deptId, deptno, dname, loc 
              FROM DEPT";

 rr:subjectMap [ rr:column "deptId"; 
                 rr:termtype "BlankNode";
                 rr:class exa:dept; 
                 rr:graph exa:DeptGraph;
                 rr:inverseExpression 
                 "{alias.}deptno = 
                  substr({alias.}deptId,length('Department')+1)"
               ];

 rr:predicateObjectMap
 [ 
   rr:predicateMap [ rr:predicate dept:deptno ]; 
   rr:objectMap    [ rr:column "deptno"; 
                     rr:datatype xsd:positiveInteger 
                   ]
 ];

 rr:predicateObjectMap 
 [ 
   rr:predicateMap [ rr:predicate dept:name ]; 
   rr:objectMap    [ rr:column "dname" ]
 ];

 rr:predicateObjectMap 
 [ 
   rr:predicateMap [ rr:predicate dept:location ]; 
   rr:objectMap    [ rr:column "loc" ]
 ];
.

Resulting RDF graph

Where are we?

• Use Cases and Requirements for Mapping Relational Databases to RDF
  Working Draft 8 June 2010 — http://www.w3.org/TR/rdb2rdf-ucr/
• R2RML: RDB to RDF Mapping Language
  Working Draft 28 October 2010 — http://www.w3.org/TR/r2rml/
• A Direct Mapping of Relational Data to RDF
  Editor's Draft 9 November 2010 — http://www.w3.org/2001/sw/rdb2rdf/directMapping/
• R2RML Test Cases
  Editor's Draft 25 August 2010 — http://www.w3.org/2001/sw/rdb2rdf/test-cases/

Next steps ...

• Finalising direct mapping (First Public Working Draft expected end of Nov 2010)
• Gathering implementation experience and work on test cases
• Updating use case document as well as the R2RML document
• Your feedback on any of these documents is highly appreciated!

Resources

• Relational Databases on the Semantic Web, Tim Berners-Lee, Design Issue Note
• Linked Data Aspects of R2RML
• Presentation of several existing RDB2RDF implementations
• From Relational Databases to the Semantic Web: New W3C Standards and Directions
  (video part 1 and video part 2 from the SemTech 2010 panel)
• Relational Database and the Semantic Web
• My first mapping from RDB to RDF using R2RML

Acknowledgements

Kudos to current and former members of the RDB2RDF Working Group:

Marcelo Arenas, Sören Auer, Samir Batla, Alexandre Bertails, Richard Cyganiak, Daniel Miranker, Souripriya Das, Alexander de Leon, Orri Erling, Ahmed Ezzat, Lee Feigenbaum, Angela Fogarolli, Enrico Franconi, Howard Greenblatt, Wolfgang Halb, Harry Halpin, Patrick Hayes, Nuno Lopes, Li Ma, Ashok Malhotra, Ivan Mikhailov, Eric Prud'hommeaux, Juan Sequeda, Seema Sundara, Ben Szekely, Edward Thomas, and Boris Villazon-Terrazas.

... as well as individuals that provided feedback and support:

Dan Connolly, Dan Brickley, Ivan Herman, Simon Reinhardt, Thomas Roessler.