3 Simple Knowledge Organization System (SKOS) Support

You can perform inferencing based on a core subset of the Simple Knowledge Organization System (SKOS) data model, which is especially useful for representing thesauri, classification schemes, taxonomies, and other types of controlled vocabulary.

SKOS is based on standard semantic web technologies including RDF and OWL, which makes it easy to define the formal semantics for those knowledge organization systems and to share the semantics across applications.

Support is provided for most, but not all, of the features of SKOS, the detailed specification of which is available at http://www.w3.org/TR/skos-reference/.

Around 40 SKOS-specific terms are included in the RDF Semantic Graph support, such as skos:broader, skos:relatedMatch, and skos:Concept. Over 100 SKOS axiomatic triples have been added, providing the basic coverage of SKOS semantics. However, support is not included for the integrity conditions described in the SKOS specification.

To perform SKOS-based inferencing, specify the system-defined SKOSCORE rulebase in the rulebases_in parameter in the call to the SEM_APIS.CREATE_ENTAILMENT procedure, as in the following example:

EXECUTE sem_apis.create_entailment('tstidx',sem_models('tst'), sem_rulebases('skoscore'));

Example 3-1 defines, in Turtle format, a simple electronics scheme and two relevant concepts, cameras and digital cameras. Its meaning is straightforward and its representation is in RDF. It can be managed by Oracle Database in the same way as other RDF and OWL data.

Example 3-1 SKOS Definition of an Electronics Scheme

ex1:electronicsScheme rdf:type skos:ConceptScheme;
ex1:cameras rdf:type skos:Concept;
   skos:prefLabel "cameras"@en;
   skos:inScheme ex1:electronicsScheme.
ex1:digitalCameras rdf:type skos:Concept;
   skos:prefLabel "digital cameras"@en;
   skos:inScheme ex1:electronicsScheme.
ex1:digitalCameras skos:broader ex1:cameras.

3.1 Supported and Unsupported SKOS Semantics

This section describes features of SKOS semantics that are and are not supported by Oracle Database.

3.1.1 Supported SKOS Semantics

All terms defined in SKOS and SKOS extension for labels are recognized. When the SKOSCORE rulebase is chosen for inference, the recognized terms include the following:


Most SKOS axioms and definitions are supported including the following: S1-S8, S10-S11, S15-S26, S28-S31, S33-S36, S38-S45, S47-S50, and S53-S54. (See the SKOS detailed specification for definitions.)

Most SKOS integrity conditions are supported, including S9, S13, S27, S37, and S46.

S52 is partially supported.

S55, S56, and S57 are not supported by default.

  • S55, the property chain (skosxl:prefLabel, skosxl:literalForm), is a subproperty of skos:prefLabel.

  • S56, the property chain (skosxl:altLabel, skosxl:literalForm), is a subproperty of skos:altLabel.

  • S57, the property chain (skosxl:hiddenLabel, skosxl:literalForm), is a subproperty of skos:hiddenLabel.chains.

However, S55, S56, and S57 can be implemented using the OWL 2 subproperty chain construct. For information about property chain handling, see Property Chain Handling.

3.1.2 Unsupported SKOS Semantics

The following features of SKOS semantics are not supported:

  • S12 and S51: The rdfs:range of the relevant predicates is the class of RDF plain literals. There is no check that the object values of these predicates are indeed plain literals; however, applications can perform such a check.

  • S14: A resource has no more than one value of skos:prefLabel per language tag. This integrity condition is even beyond OWL FULL semantics, and it is not enforced in the current release.

  • S32: The rdfs:range of skos:member is the union of classes skos:Concept and skos:Collection. This integrity condition is not enforced.

  • S55, S56, and S57 are not supported by default, but they can be implemented using the OWL 2 subproperty chain construct, as explained in Supported SKOS Semantics.

3.2 Performing Inference on SKOS Models

Performing inference on a SKOS model is similar to performing inference on a semantic model.

To create an SKOS model, use the same procedure (SEM_APIS.CREATE_SEM_MODEL) as for creating a semantic model. You can load data into an SKOS model in the same way as for semantic models.

To infer new relationships for one or more SKOS models, use the SEM_APIS.CREATE_ENTAILMENT procedure with the system-defined rulebase SKOSCORE. For example:

EXECUTE sem_apis.create_entailment('tstidx',sem_models('tst'), sem_rulebases('skoscore'));

The inferred data will include many of the axioms defined in the SKOS detailed specification. Like other system-defined rulebases, SKOSCORE has no explicit rules; all the semantics supported are coded into the implementation.

3.2.1 Validating SKOS Models and Entailments

You can use the SEM_APIS.VALIDATE_ENTAILMENT and SEM_APIS.VALIDATE_MODEL procedures to validate the supported integrity conditions. The output will include any inconsistencies caused by the supported integrity conditions, such as OWL 2 propertyDisjointWith and S52.

Example 3-2 validates an SKOS entailment.

Example 3-2 Validating an SKOS Entailment

set serveroutput on
  lva mdsys.rdf_longVarcharArray;
  idx int;
  lva := sem_apis.validate_entailment(sdo_rdf_models('tstskos'), sem_rulebases('skoscore'));
  if (lva is null) then
    dbms_output.put_line('No conflicts');
  for idx in 1..lva.count loop
    dbms_output.put_line('entry ' || idx || ' ' || lva(idx));
  end loop;
  end if;

3.2.2 Property Chain Handling

The SKOS S55, S56, and S57 semantics are not supported by default. However, you can add support for them by using the OWL 2 subproperty chain construct.

Example 3-3 inserts the necessary chain definition triples for S55 into an SKOS model. After the insertion, an invocation of SEM_APIS.CREATE_ENTAILMENT that specifies the SKOSCORE rulebase will include the semantics defined in S55.

Example 3-3 Property Chain Insertions to Implement S55

INSERT INTO tst VALUES(sdo_rdf_triple_s('tst','<http://www.w3.org/2004/02/skos/core#prefLabel>', '<http://www.w3.org/2002/07/owl#propertyChainAxiom>', '_:jA1'));
INSERT INTO tst VALUES(sdo_rdf_triple_s('tst','_:jA1', '<http://www.w3.org/1999/02/22-rdf-syntax-ns#first>', '<http://www.w3.org/2008/05/skos-xl#prefLabel>'));
INSERT INTO tst VALUES(sdo_rdf_triple_s('tst','_:jA1', '<http://www.w3.org/1999/02/22-rdf-syntax-ns#rest>', '_:jA2'));
INSERT INTO tst VALUES(sdo_rdf_triple_s('tst','_:jA2', '<http://www.w3.org/1999/02/22-rdf-syntax-ns#first>', '<http://www.w3.org/2008/05/skos-xl#literalForm>'));
INSERT INTO tst VALUES(sdo_rdf_triple_s('tst','_:jA2', '<http://www.w3.org/1999/02/22-rdf-syntax-ns#rest>', '<http://www.w3.org/1999/02/22-rdf-syntax-ns#nil>'));