如何使用SPARQL查询RDF三元组列表？

Question

I have an RDF database (AFAIK) where I store Java objects which are mapped to RDF triples.

What I want to do now is query for a list of all objects of a specific type.

I imagine something like this, where I have put in my imagination of the syntax:

PREFIX xyz: <http://foo.com/myclassname#>
SELECT *
WHERE {
  xyz:* name ?name
}

I want to say that I want all triples which represent names of the subject with type myclassname. How to do this correctly?

Right now I can't give you error messages, because I do not have a running system, but I'm sure I do not understand SPARQL queries well enough, so please shed some light on this.

Answer 1

SPARQL queries are generally in the form ?subject ?predicate ?object . So you need to run a simple query like this to know if you need to put your restriction in the place of ?Predicate or you need to add another triple ( ?object ?x ?y ) to your query. Please refer to SPARQL for begginers for more examples.

Your answer in short is that yes, you can query based on a specific type, but you need to know where to put this type restriction.

So for example in DBpedia if you want to get the labels of everything you can say:

select distinct *
where 
    {?s rdfs:label ?o}
LIMIT 100

If you want this label to be a name, you need to specify the type of your object:

select distinct *
where 
    {?s rdfs:label ?o.
    ?s a dbpedia-owl:Person}
 LIMIT 100

Answer 2

The exact syntax of the SPARQL query depends on how exactly your data is modeled in RDF. Let's assume that objects of a class MyClass are represented as follows in your RDF database, and that they have a name property my:name :

@prefix my: <http://example.org/my/> .

my:object1 a my:MyClass . 
my:object1 my:name "object 1" .

A query to get back all objects and their names modeled in this exact fashion would look like this:

PREFIX my: <http://example.org/my/>

SELECT ?x ?name
WHERE { 
     ?x rdf:type my:MyClass ;
        my:name ?name .
}

SPARQL queries work as graph pattern matchers : the query looks for any combination of triples matching the pattern specified in the WHERE-clause: so we're looking for two triples, one expressing that the rdf:type property has the value my:MyClass , and one expressing that another triple with the same subject should have a property called my:name , of which we want back the value (bound to the variable ?name ).

As you can see, it's dependent on your specific RDF model: in your database, the name of the class ( my:MyName ) and the name of the property ( my:name ) are probably different. So you will need to find out how exactly your data is modeled as RDF before you can write a query that works in your specific case.

Edit as an aside: SPARQL has a shorthand notation for the property rdf:type , which is a . So the bit ?x rdf:type my:MyClass is often written as ?xa my:MyClass . I wanted to stress the fact that you are querying triples/properties, which is why I used the actual property name.