Question

I’m new to this entire semantics stuff, and i’ve been plunged into a project which requires me to use the SPARQL endpoint from dbpedia to retrieve information about certain things such as a City for example. The idea is that my application collects a few of these keywords (which are predefined, such as City, Country & Region) and rely on dbpedia to get some sensible information about this.

After fiddling around with the SPARQL endpoint for some time, I came up with the following query:

PREFIX dbo: <http://dbpedia.org/ontology/> 
PREFIX prop: <http://dbpedia.org/property/>

SELECT ?city
WHERE {
?city prop:name "X"@en; 
a dbo:PopulatedPlace
}

Where X stands for the name of the city that I’m trying to get information about. When I run this query against the SPARQL endpoint, I receive a URI in a RDF structure that points to a RDF store on DBPedia (at least that’s how I interpret it, please correct me if I’m wrong)

The returned result for the City “Antwerpen” looks like this:

<rdf:RDF><rdf:Description rdf:nodeID="rset"><rdf:type rdf:resource="http://www.w3.org/2005/sparql-results#ResultSet"/><res:resultVariable>city</res:resultVariable><res:solution rdf:nodeID="r0"><res:binding rdf:nodeID="r0c0"><res:variable>city</res:variable><res:value rdf:resource="http://dbpedia.org/resource/Antwerp"/></res:binding></res:solution></rdf:Description></rdf:RDF>

I’m using Ruby on Rails with the RDF for Ruby gem to work with RDF data. But honestly, I’m clueless on how to actually work with this information. When I follow the link, I seem to be receiving an RDF store that contains all information about the city I queried.

Am I right in that I probably need to point my Code to the URI received and parse our the information I desire? Or should it be possible for example to select some information directly through the SPARQL endpoint? Like for example the description & demographic data?

I know this is a pretty vague question, but I’m trying my best to get the hang of this technology, and looking for some examples to help me better understand it.

Answer 1

It’s probably easier to get started by getting all the info you want directly from the SPARQL endpoint. You can use a query like this:

SELECT ?city ?property ?value
WHERE {
    ?city prop:name "X"@en; 
          a dbo:PopulatedPlace;
          ?property ?value .
}

This will return a bunch of properties and values that describe the city in some detail and it will give you a flavour for the kind of information that you can get.

A next step would be to query specifically for the properties you’re interested in:

SELECT *
WHERE {
    ?city prop:name "Galway"@en; 
          a dbo:PopulatedPlace .
    OPTIONAL { ?city dbo:populationUrban ?pop }
    OPTIONAL { ?city foaf:homepage ?homepage }
    OPTIONAL { ?city geo:lat ?lat }
    OPTIONAL { ?city geo:long ?long }
}

I’m putting each of the triple patterns into an OPTIONAL block because some cities may not have all of the information available. Without the OPTIONAL block, you’d only get a result if all the triple patterns match for the city.