Executing queries

Let's say we have an RDF.ex graph like this:

graph = RDF.Turtle.read_string! """
  @prefix foaf:  <http://xmlns.com/foaf/0.1/> .
  _:a  foaf:name   "Johnny Lee Outlaw" .
  _:a  foaf:mbox   <mailto:jlow@example.com> .
  _:b  foaf:name   "Peter Goodguy" .
  _:b  foaf:mbox   <mailto:peter@example.org> .
  _:c  foaf:mbox   <mailto:carol@example.org> .

We can execute the following SPARQL query:

query = """
  PREFIX foaf:   <http://xmlns.com/foaf/0.1/>
  SELECT ?name ?mbox
    { ?x foaf:name ?name .
      ?x foaf:mbox ?mbox }

like this:

SPARQL.execute_query(graph, query)

This will return a SPARQL.Query.Result struct which contains the results under the results field as a list of maps with the bindings of the solutions.

  results: [
    %{"mbox" => ~I<mailto:peter@example.org>, "name" => ~L"Peter Goodguy"},
    %{"mbox" => ~I<mailto:jlow@example.com>, "name" => ~L"Johnny Lee Outlaw"}
  variables: ["name", "mbox"]

The list of results for a single variable can be fetched with the SPARQL.Query.Result.get/2 function.

SPARQL.execute_query(graph, query) 
|> SPARQL.Query.Result.get(:mbox)

If SPARQL.execute_query/2 is used to execute a CONSTRUCT query, it will return an RDF.Graph:

iex> SPARQL.execute_query graph, """
...>  PREFIX foaf:   <http://xmlns.com/foaf/0.1/>
...>  PREFIX schema: <http://schema.org/>
...>  CONSTRUCT   
...>    { ?x schema:name ?name ;
...>         schema:email ?mbox }
...>  WHERE
...>    { ?x foaf:name ?name ;
...>         foaf:mbox ?mbox }
...>  """
#RDF.Graph{name: nil
             ~L"Peter Goodguy" 
             ~L"Johnny Lee Outlaw"}

The SPARQL.execute_query/2 function converts a given query string implicitly to a SPARQL.Query struct. If you intend to execute the query multiple times it's better to do this step on your own with the SPARQL.query/1 function and pass the interpreted query directly to SPARQL.execute_query/2, in order to not parse the query on every execution.

query = SPARQL.query """
  PREFIX foaf: <http://xmlns.com/foaf/0.1/>
  SELECT ?name ?mbox
    { ?x foaf:name ?name .
      ?x foaf:mbox ?mbox }

SPARQL.execute_query(graph, query)

Default prefixes

By default the configured RDF.default_prefixes/0 are defined before every query. See the section about Managing prefixes for more on that. So, unless you've configured additional default_prefixes or set use_standard_prefixes to false, you'll get these prefixes implicitly and can use them on every query without having to define them all the time.

PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>

You can also provide your own query-specific default prefixes with the default_prefixes option on the SPARQL.query/1 function. Setting this option to nil or an empty map disables the default_prefixes just on a single query.

The prefixes you're using in your query are also used for the RDF.Graph build with CONSTRUCT queries, so it might be useful to also specify additional prefixes in CONSTRUCT queries, which are not directly used in your query but in the nodes of the resulting graph (although you can also add those later programmatically with RDF.Graph.add_prefixes/2).