SPARQL aggregate query example using Ruby SPARQL::Client gem

aggregate_sparql_query.rb

require 'rubygems'
require 'sparql/client'

ENDPOINT_URL = "http://localhost:3030/data/query"

query =
  SPARQL::Client.new(ENDPOINT_URL, :method => 'post') 
    .select(:person, :count => {:other_person => :count} )
    .where([:person, RDF::FOAF.knows, :other_person])
    .group(:person)
    .order("DESC(?count)")

# => SELECT ?person ( COUNT(?other_person) AS ?count ) WHERE { ?person <http://xmlns.com/foaf/0.1/knows> ?other_person . } GROUP BY ?person ORDER BY DESC(?count)

query.solutions.map(&:to_hash)
# => [{:person=>#<RDF::URI:0xac81b8(http://example.org/alice)>, 
#  :count=>
#   #<RDF::Literal::Integer:0xac9a7c("3"^^<http://www.w3.org/2001/XMLSchema#integer>)>},
# {:person=>#<RDF::URI:0xa99340(http://example.org/bob)>,
#  :count=>
#   #<RDF::Literal::Integer:0xa97edc("1"^^<http://www.w3.org/2001/XMLSchema#integer>)>},
# {:person=>#<RDF::URI:0x98c4fc(http://example.org/charlie)>,
#  :count=>
#   #<RDF::Literal::Integer:0x98fee0("1"^^<http://www.w3.org/2001/XMLSchema#integer>)>}]