Transformation of OWL2 axioms into SPARQL
Semantic queries can also be defined in the ontology with the help of the OWL2 axiomatic. They can be:
- - mandatory queries for the proper function of certain system operations like the search for a certain type of medium that responds to certain criteria;
- - search operations that are often requested by the user as for instance the search for an analysis containing a certain type of stratum that itself contains a certain type of segment;
- queries responding to the description patterns allowing the objects/individuals that were defined according to these patterns to be found.
As described in section 4.3.2, an OWL2 axiomatic defines complex anonymous classes whose properties are recursively subject to several property/value restrictions. The OWL2 resolution method presented in section 4.3.3 allows the corresponding SPARQL queries to be generated. Contrary to RDF structuring, the axiomatic representation of a semantic query offers more expressivity. In fact, the different SPARQL operators are natively implemented in an OWL2 expression. However, this representation does not allow cycles to be specified.
The knowledge graph editing interfaces presented in section 4.3.7 are reused to edit the semantic queries. Figure 4.14 shows the graph that corresponds to the search for “all time segments in which there is a musical rendition of a song”.
Figure 4.14. Graph of a semantic query
This graph is transformed into a SPARQL query and executed by the semantic search engine. Figure 4.15 shows all possible combinations (in the knowledge base) of segment, rendition and song that reproduce the query structure.
Figure 4.15. Results of a semantic query