ISSUE-3: multiple values returned from a reference
multiple values returned from a reference
- State:
- RAISED
- Product:
- Raised by:
- Anastasia Dimou
- Opened on:
- 2019-07-25
- Description:
- Vladimir questions if are carml:multiReference and carml:multiTemplate which are proposed by Pano Maria needed or rml:reference and r:template suffice?
Anastasia thinks that the RML specification should not restrict the number of values that are returned for a certain rml:reference or a reference included in rr:template.
This comes as a consequence of including other formats. R2RML refers to tables in relational databases. Thus, it is normal to expect that one value would be returned for a certain reference (rr:column).
However, RML works with other formats, including eg XML. There, an XPath expression might be validated over multiple elements and, thus, multiple values are returned. RML needs to support these cases by generating multiple RDF terms, one for each returned value.
- Related Actions Items:
- No related actions
- Related emails:
- kg-construct-ISSUE-3: multiple values returned from a reference (from sysbot+tracker@w3.org on 2019-07-25)
Related notes:
Hi,
I agree with Anastasia on the fact that the number of values generated by an rml:reference or rr:template should not be restricted to 0 or 1. Indeed, unlike relational databases, XPath or JSONPath expressions may typically return multiple values, thus generating multiple RDF terms.
This is why, in xR2RML, we described this behaviour explicitly in the specification. See here: http://i3s.unice.fr/~fmichel/xr2rml_specification.html#_Toc466307470
This goes through the definition of a term map iteration that can occur within a triples map iteration.
Overall, any term map can generate multiple RDF terms (may they be subject, predicate, object or graph maps). The resulting triples are the cartesian product of all those terms.
Franck.
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