title: How to query a column which value is null in JPA?
body: <p>I am using JPA namedQuery to select data from DB. </p>
  
  <pre><code> @NamedQuery(name = "Concept.findByRefTableNull", query = "SELECT c FROM Concept c WHERE c.conceptName = :conceptName and c.refTable = :refTable"),
  
  ///
   List&lt;Concept&gt; attributeList 
                  = em.createNamedQuery("Concept.findByRefTableNull")
                  .setParameter("conceptName", "student").
                  setParameter("refTable", null).
                  getResultList();
  
          System.out.println(attributeList.size()); //return 0
  </code></pre>
  
  <p>The List size is 0, but I am sure it should have records. The reason is the refTable. How to query a column which value is null in JPA ?</p>
  
  <p>Thanks.</p>