Join between non-RDF and RDF data on the subject position

[LorenzBuehmann]

Currently, RDF data is parsed as URI and put into a DataFrame with (shortened) URIs.

Consider the N-Triples

<http://example.org/a1> <http://example.org/p1> <http://example.org/b1> .
<http://example.org/a1> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://example.org/A> .

and the mapping

<#AMapping>
	rml:logicalSource [
		rml:source "/tmp/datalake-test/a.nt";
		nosql:store nosql:rdf
	];
	rr:subjectMap [
		rr:template "{id}";
		rr:class ex:A
	];

	rr:predicateObjectMap [
		rr:predicate ex:p1;
		rr:objectMap [rml:reference "example.org/p1"]
	] .

the data will be converted to this DataFrame

root
|-- id: string (nullable = true)
|-- example.org/p1: string (nullable = true)

+--------------+--------------+
|id |example.org/p1|
+--------------+--------------+
|example.org/a1|example.org/b1|
+--------------+--------------+

The problem now is, any other data is just handled by the plain values contained the the corresponding datasource, i.e. it's never handled internally as URI as one would expect by the RML mappings.

Consider the CSV file

nr,p2
b1,c1
b2,c2
b3,c3

and the mapping

<#BMapping>
	rml:logicalSource [
		rml:source "/tmp/datalake-test/b.csv";
		nosql:store nosql:csv
	];
	rr:subjectMap [
		rr:template "http://example.org/{nr}";
		rr:class ex:B
	];

	rr:predicateObjectMap [
		rr:predicate ex:p2;
		rr:objectMap [rml:reference "p2"]
	] .

the DataFrame will just be

root
 |-- nr: string (nullable = true)
 |-- p2: string (nullable = true)

+---+---+
|nr |p2 |
+---+---+
|b1 |c1 |
|b2 |c2 |
|b3 |c3 |
+---+---+

Clearly, any join would fail and result in an empty DataFrame:

prefix ex: <http://example.org/>

select * where {
  ?s a ex:A ;
       ex:p1 ?o .
   ?o ex:p2 ?o1 .
}

[mnmami]

Hi Patrick,

Thanks for creating an issue for this interesting situation when one needs to query RDF and non-RDF data in the same query. Since the subject (in RDF) is always a URI, one cannot join non-RDF data with RDF data on the subject position.

For this to work, we need to incorporate a way (both at RML mappings and Squerall code) to either extract a plain value from the URI in the RDF data, or create a URI from a non-RDF plain value (by attaching a full namespace).