title: JPA where clause any
body: <p>In JPA, the query is:</p>
  
  <pre><code>Query q = entityManager.createQuery("select o from Product o WHERE o.category = :value");
  q.setParameter("category", category);
  </code></pre>
  
  <p>How can I set category to any category in JPA? So if the null category passed, I simple ignore the category parameter, select all products.</p>