Conditional creation of property in R2RML

Question

I'm playing around with R2RML and I was wondering if I can create a property depending on the content of a RDB table cell.

The D2RQ mapping language has d2rq:condition that can handle that.

e.g.:

if value in column/table cell 'name' is 'abc' create property 'abc'

rr:predicateObjectMap [
    rr:predicate ex:abc
    rr:objectMap [ 
        rr:column "name"; 
        rr:datatype xsd:string; 
        # equivalent for d2rq:condition "name='abc'"
    ];
]

if value in column/table cell 'name' is 'xyz' create property 'xyz'

rr:predicateObjectMap [
    rr:predicate ex:xyz
    rr:objectMap [ 
        rr:column "name"; 
        rr:datatype xsd:decimal; 
        # equivalent for d2rq:condition "name='xyz'"
    ];
];

I couldn't find any suggestion in W3C's R2RML Recommendation.

Any ideas? :-)

---

Update:

I had the idea of using rr:sqlQuery e.g.

rr:SQLQuery """
   select (case TABLENAME.COLUMNNAME
      when 'this' then 'propertyOne'
      when 'that' then 'propertyTwo'
      end) as VARIABLE_PREDICATE
   from TABLENAME """;

and apply it to a rr:predicate or rr:predicateMap with

rr:predicateObjectMap [
        rr:predicateMap [ rr:template "ex:{VARIABLE_PREDICATE}" ];
        rr:objectMap [ rr:column "COLUMNNAME"; ];
];

But that didn't work. I guess predicateMaps can be rr:constants only and not rr:templates :( . At least the W3C Recommendation just shows constants within predicateMap.

Still searching for a solution... :/

P.S. I'm disappointed that a proprietary language like d2rq seems to be more powerful (at this point).

Answer 1

R2RML doesn't have conditional properties (like in D2RQ). The design was done on purpose in order not to complicate the language. Any type of "complex" mapping requires SQL.

A solution is the following:

@prefix rr: <http://www.w3.org/ns/r2rml#>.

<#Mapping> a rr:TriplesMap;
    rr:logicalTable [ rr:SQLQuery """
        select id, COLUMNNAME, (case TABLENAME.COLUMNNAME
        when 'this' then 'http://ex.com/propertyOne'
        when 'that' then 'http://ex.com/propertyTwo'
        end) as VARIABLE_PREDICATE
        from TABLENAME """; ];
    rr:subjectMap [
       rr:template "http://ex.com/foo/{id}";
   ];

   rr:predicateObjectMap [
      rr:predicateMap [ rr:column "VARIABLE_PREDICATE" ];
      rr:objectMap [ rr:column "COLUMNNAME" ];
   ].