Supporting Relational Database Joins for Generating Literals in R2RML

Christophe Debruyne

Keywords: R2RML, Knowledge Graph Generation, Outer-joins, Joins

Abstract: Since its publication, R2RML has provided us with a powerful tool for generating RDF from relational data, not necessarily manifested as relational databases. R2RML has its limitations, which are being recognized by W3C's Knowledge Graph Construction Community Group. That same group is currently developing a specification that supersedes R2RML in terms of its functionalities and the types of resources it can transform into RDF--primarily hierarchical documents. The community has a good understanding of problems of relational data and documents, even if they might need to be approached differently because of their different formalisms. In this paper, we present a challenge that has not been addressed yet for relational databases--generating literals based on (outer-)joins. We propose a simple extension of the R2RML vocabulary and extend the reference algorithm to support the generation of literals based on (outer-)joins. Furthermore, we implemented a proof-of-concept and demonstrated it using a dataset built for benchmarking joins. While it is not (yet) an extension of RML, this contribution informs us how to include such support and how it allows us to create self-contained mappings rather than relying on less elegant solutions.