Copia

I recently read Morten Frederiksen's blog entry about implementing Source Content Descriptions as SPARQL queries in Redland and was quite interested. Especially the consideration that such queries could be automatically generated and the set of these queries you would want to ask is small and straight forward. Even more interesting was Morten's step-by-step walk-thru of how such queries would be translated to SQL queries on a Redland Triple store sitting on top of MySQL (my favorite RDBMS deployment for 4RDF as well).

However, I couldn't help but wonder how such a set of queries would be expressed in Versa (in my opinion, a language more aligned with the data model it queries than it's SQL-RDQL counter-parts). So below was my attempt to port the queries into versa:

Classes used in the store

SPARQL

SELECT DISTINCT ?Class
WHERE { ?R rdf:type ?Class }

Versa

set(all() - rdf:type -> *)

Predicates that are used with instances of each class

SPARQL

SELECT DISTINCT ?Class, ?Property
  WHERE { ?R rdf:type ?Class .
        OPTIONAL { ?R ?Property ?Object .
                   FILTER ?Property != rdf:type } }

Versa

difference(
  properties(set(all() - rdf:type -> *)),
  set(rdf:type)
)

Do all instances of each class have a statement with each predicate?

It wasn't clear to me if the intent was to check if all classes have a statement with each predicate as specified by an ontology or to just count how many properties each class instance has. The latter interpretation is the one I went with (it's also simpler). This particular query will return a list of lists, each inner list consisting of two values: the URI of a distinct class instance and the number of distinct properties described in a statements about it (except rdf:type)

Versa

distribute(
  set(all() |- rdf:type -> *),
  '.',
  'length(
    difference(
      properties(.),
      set(rdf:type)
    )
  )'
)

Is the type of object in a statement with each class/predicate combination always the same?

I wasn't clear on the intent of this query, either. I wasn't sure if he meant to ask this of the combination with all predicates defined in an ontology or all predicates on class instances in the graph being queried.

But there you have it.

NOTE: The use of the set function was in order to guarantee that only distinct values were returned and may have been used redundantly with functions and expressions that already account for duplication.

[Uche Ogbuji]