OCHRE and the Semantic Web

Writing a SPARQL query template will require both expertise on the Query Language, and research as to the vocabulary (properties, entities, terminology) of the domain of interest. Many linked-data sites provide extensive documentation and a variety of examples. But it is up to someone who knows how to write SPARQL queries to prepare an appropriate query template that will be understood by OCHRE.

This is not the place to explain the syntax of SPARQL, except to say it is modeled on the principle of referencing a "triple" composed of a subject-predicate-object. E.g. find an item (subject) whose label (predicate) is "Quercus" (object).

There are several rules and options to be aware of when preparing a SPARQL template for OCHRE:

• The template must contain the special code <Name> as the object of a predicate, or as the target of some appropriate string-matching request. When OCHRE sends the query to the SPARQL engine, it will substitute in the Name of the OCHRE item in place of the "<Name>" code. In this example, the WHERE clause will become:

                    WHERE { ?item ? label "Quercus"@en .

• The query must be designed to return any of a ?subject, ?item, ?id, or ?identifier as a keyword. The return value of one of these "subject" items must contain the identifying code that uniquely identifies the entity being fetched. OCHRE will append the value of this identifier to the Entity URI to create the official, stable URI for the item being represented.
• The query should return some descriptive content -- a definition, a preferred label, a term, a description, etc. -- based on the vocabulary of the domain to provide some explanatory content for the returned values. OCHRE will create a descriptive string of such content.
• The query may contain a return value of ?image. Images are fetched in a variety of ways, based on the domain and the data set; the query syntax would need to support this. If one or more images are returned, OCHRE will retain them as part of the Thesaurus link. In this case, the Semantic Web icon is lit more brightly with a cyan edge to visually indicate the presence of an image. Images will be fetched on-the-fly when needed for a View.
• The query may contain return values of ?lat and ?long (alternately, ?latitude, ?lon, ?longitude). If an appropriate pair of coordinates is recognized by OCHRE, and if that item's metadata Coordinates are blank, OCHRE will assign the looked-up coordinates to the item and save these. Alternately, the query may return ?coordinates, and OCHRE will parse the value into a latitude/longitude pair, expecting data in the format: Point(lon lat) (as given by Wikidata's P625 "coordinate location" for example).
• The return format must be RDF/XML. OCHRE provides a field to enter a Output format instruction if such is needed to request RDF/XML as the SPARQL return format from the given domain. SPARQL engines often return RDF/XML by default making this unnecessary in many cases.