title: Select subjects and rdf:type in a SPARQL query
body: <p>I have a SPARQL query shown below to select subjects having specific labels I defined from DBpedia.</p>
  
  <pre><code>SELECT ?subject
  WHERE {
    ?subject rdfs:label ?label.
    FILTER ( regex (?label,"^json$", "i") )
  }
  </code></pre>
  
  <p>It works for me.</p>
  
  <p>And now I am trying to retrieve <code>rdf:type</code> of each subject by this query.</p>
  
  <pre><code>SELECT ?subject ?type
  WHERE {
    ?subject rdfs:label ?label;
    rdf:type ?type.
    FILTER ( regex (?label,"^json$", "i") )
  }
  </code></pre>
  
  <p>But I get an empty result from this.</p>
  
  <p>If replacing <code>?subject</code> with a specific resource, it works again.</p>
  
  <pre><code>SELECT ?type
  WHERE {
    &lt;http://dbpedia.org/resource/JSON&gt; rdf:type ?type.
  }
  </code></pre>
  
  <p>Cannot figure out what happens.
  How can I get results as expected?</p>