SPARQL - select from skos:category - Virtuoso 3700

问题:

I have problem with SPARQL. I want to select something from category. For example subjects. I make query like this in http://dbpedia.org/snorql.

SELECT ?category ?subject WHERE
    {
    ?category a skos:Concept .
    ?category skos:Concept: American_punk_rock_guitarists.
    ?category dct:subject ?subject .
    } LIMIT 1000

I have error Virtuoso 37000. I don't understand why.

P.S. Is it good book for beginnier in SPARQL - Learning SPARQL, 2nd Edition Querying and Updating with SPARQL 1.1 ?

回答1:

You have at least one syntax error: the second colon (:) in the second triple.

Semantically... I don't really know the classes or predicates in dbpedia... but can skos:Concept be both a type and a predicate?

I wrote you a valid query that returns 10 members of the category "American_punk_rock_guitarists"

I put this together by going to dbpedia's faceted free text search and familiarizing myself with the concept of American punk rock guitarists, specifically Joey Ramone

 prefix dbpcat: <http://dbpedia.org/resource/Category:>

    SELECT  ?subject ?category
    WHERE
      { values ?category { dbpcat:American_punk_rock_guitarists  } .
      ?subject dct:subject ?category }
    LIMIT   10

回答2:

In terms of the Virtuoso 37000 error, your mistakes were the following:

1. You had : after skos:Concept in line 2. and you were missing a prefix for "American_punk_rock_guitarists".

2. American_punk_rock_guitarists should have been dbpcat:American_punk_rock_guitarists. Though this is going to give you an empty result, the syntactically correct version or your query is:

   prefix dbpcat: <http://dbpedia.org/resource/Category:>
   
   SELECT ?category ?subject WHERE
       {
       ?category a skos:Concept .
       ?category skos:Concept dbp:catAmerican_punk_rock_guitarists.
       ?category dct:subject ?subject .
       } LIMIT 1000
   

   This returns an empty set because of the following:

a. skos:Concept is a class, not a property. In terms of {?Subject ?Predicate ?Object}, you will only see skos:Concept as a ?Subject and/or ?Object in a proper SPARQL query [1]. An easy way to differentiate classes and properties is that classes are supposed to start with a capital letter (ex: skos:Concept), and properties (predicates) should start with a lowercase letter (ex: rdf:type).

b. dbpcat:American_punk_rock_guitarists is of type "skos:Concept". If you look at the DBPedia page for this entity (?subject), you will see this in the "rdf:type" row. Look further and you'll see that this concept is a dct:subject of various other entities [2].

In addition to @Mark Miller's query this one will also provide results:

prefix dbpcat: <http://dbpedia.org/resource/Category:>

    SELECT  ?subject ?category
    WHERE
      { ?subject dct:subject ?category.
        ?category a skos:Concept.
        filter(?category = dbpcat:American_punk_rock_guitarists).

      }
    LIMIT   10

skos:Concept isn't needed here, solely as an example of how it would be used if you were looking for something such as "all ?category of type - skos:Concept".

Links:

1. https://www.w3.org/2009/08/skos-reference/skos.html#Concept
2. http://dbpedia.org/resource/Category:American_punk_rock_guitarists