R2RML: RDB2RDF Mapping Language

In this document, examples assume the following namespace prefix bindings unless otherwise stated:

The R2RML language includes IRIs, a subset of RDF URI References that omits spaces.

The RDFTermMap class reprersents the description of mapping to any RDF term: IRIs, blank nodes, or literals. This has two main components: mapping to an RDF property and mapping to an object value (to be associated with the property).

This section defines the RDF properties that have the RDFTermMap class as their domain.

rr:property rdfs:range rdf:Resource .

This property specifies the RDF property component of the RDFTermMap instance.

rr:column rdfs:range xsd:string .

This property specifies the object value component of the RDFTermMap instance. Specifically, the column name of the logical table that will be used for obtaining the object value.

rr:inverseExpression rdfs:range xsd:string .

This optional property specifies an expression that allows, at query processing time, use of indexes on any (underlying) relational table when accessing based on a value of a column (defined as an expression) in the logical table.

For example, for the deptId column in the logical table shown in Section 3.3.1.1, the inverse expression could be defined as follows:

"{alias.}deptno = substr({alias.}deptId,length('dept')+1)"

to facilitate use of an index on the deptno column of the dept table. Note that the actual alias (say d) used for the dept table is used to expand {alias.} (to d.).

rr:datatype rdfs:range rdf:Resource .

This property specifies the datatype for the object value component of the RDFTermMap instance.

rr:language rdfs:range xsd:string .

This property specifies the language for the object value component of the RDFTermMap instance. This property is applicable only if the datatype of the object value is of type xsd:string.

rr:columnGraph rdfs:range rr:SubjectTermMap .

This property specifies the RDF named graph for the triples that is constructed with the (property, object) pair of the RDFTermMap.

rr:constantValue rdfs:range xsd:string .

This property specifies the optional constant value for the object component of the RDFTermMap instance.

rr:isRDFType rdfs:range xsd:boolean .

This property specifies if the object value component of the RDFTermMap instance is also a value for the rdf:type property.

rr:isComputed rdfs:range xsd:boolean .

This property specifies if the object value component of the RDFTermMap instance is a computed property.

This section enumerates the subclasses of the RDFTermMap class. These classes describe the various types of RDF terms namely IRIs, blank nodes, and literals.

The RDFSubjectTermMap class represents the mapping to any RDF term that can appear as subject of an RDF triple: IRIs or blank nodes.

rr:RDFSubjectTermMap rdfs:subClassOf rr:RDFTermMap .

The IRIMap class represents the mapping to a RDF Resource.

rr:IRIMap rdfs:subClassOf rr:RDFSubjectTermMap .

The bNodeMap class represents the mapping to a RDF blank node.

rr:bNodeMap rdfs:subClassOf rr:RDFSubjectTermMap .

The LiteralMap class represents the mapping to a RDF literal.

rr:LiteralMap rdfs:subClassOf rr:RDFTermMap .

Table2TriplesMap is an RDFS class that allows specification of a mapping of the rows in a logical table, represented by a SQL query, or the name of a table or view and its owner, into RDF triples. Properties of a Table2TriplesMap instance allows specification of various aspects of the mapping such as, mapping of (some of) the columns in the logical table to RDF properties, name of the RDF graph(s) that would store the RDF triples, and so on.

This section defines the RDF properties that have the Table2TriplesMap class as their domain.

rr:logicalTable rdfs:range xsd:String .

This property specifies the logical table (i.e., a SQL query or a table or view name plus its owner name) whose rows are mapped to RDF triples by this Table2TriplesMap instance. Note that each row in the logical table is identified as a resource and is used as the subject for the RDF triples.

The following example speifies that the Table2TriplesMap instance Table2TriplesMap1 maps the rows in the logical table defined by the SQL string to RDF triples.

<#Table2TriplesMap1> rr:logicalTable 
                                """Select 'dept' || deptno AS deptId
                                        , deptno
                                        , dname
                                        , loc
                                   from dept""" .

The cardinality of this property, for a Table2TriplesMap instance, must be exactly 1. That is, a Table2TriplesMap instance must have exactly one value for this property.

rr:class rdfs:range rdfs:Class .

This property specifies the RDFS class associated with a Table2TriplesMap instance. In the generated RDF data, resource corresponding to each row in the logical table is an instance of this RDFS class.

In the following example, Table2TriplesMap1 is associated with the the RDFS class xyz:dept using rr:class. This leads to creation of xyz:dept as an RDFS class in the RDFS schema generated from the mapping specification and in the generated RDF data, RDF resource corresponding to each row in the logical table associated with Table2TriplesMap1 is an instance of the xyz:dept class.
[@@ We need to discuss this to make it optional to generate xyz:dept as an RDFS class in the generated RDFS schema.]

<#Table2TriplesMap1> rr:class xyz:dept .

rr:tableGraphMap rdfs:range rdf:Resource .

This property specifies the graph (IRI or blank node) that would contain all the RDF triples in a Table2TriplesMap instance.

In the following example, all the RDF triples in Table2TriplesMap1 would be stored in the RDF named graph xyz:DeptGraph.

<#Table2TriplesMap1> rr:tableGraphMap xyz:DeptGraph .

rr:subjectMap  rdfs:range rr:RDFSubjectTermMap .

This property specifies the mapping to obtain the IRI or blank node that is used as the subject of all the RDF triples generated from one row of the table.

rr:propertyObjectMap rdfs:range rr:RDFTermMap .

This property specifies the mapping to obtain the (property, object) pair for each RDF triple corresponding to a column and its values in the table.

rr:computedPropertyMap rdfs:range rr:IRIMap .

This property specifies the mapping to obtain the property IRIs from a column in the table. This mapping also contains a property name that identifies this computedPropertyMap and this property name is used in a rr:propertyObjectMap to form the association. The property IRI generated (from the column identified in the rr:computedPropertyMap) for each row, along with the value of the column identified in the rr:propertyObjectMap, are used to create the (property, object) pairs.

rr:rowGraphPropertyMap rdfs:range rr:SubjectTermMap .

This property specifies the mapping to obtain the IRI or blank node that identifies the RDF named graph for all the RDF triples generated from one row in the table. The minimum cardinality for this property is zero and no restriction on maximum cardinality. If for a row, the generated graph IRI or blank node label is NULL, then the triples generated from that row go to an unnamed graph.

rr:foreignKeyMap rdfs:range rr:ForeignKey .

This property specifies the mapping to obtain the property, and a join condition that can be used to retrieve the object (from the parent Table2Triples instance), for the generated RDF triple. This property would typically correspond to a foreign key definition in the table.