title: How can I order by NULL in DQL?
body: <p>I'm building an app using Symfony2 framework and using Doctrine ORM. I have a table with airlines for which some IATA codes are missing. I'm outputting a list, ordered by this IATA code, but I'm getting the undesirable result that the records with null IATA codes are sorted at the top.</p>
  
  <p>In MySQL this is simple enough to do, with <code>ORDER BY ISNULL(code_iata), code_iata</code> but I'm clueless as to what the equivalent would be for DQL. I tried </p>
  
  <pre><code>$er-&gt;createQueryBuilder('airline')-&gt;orderBy('ISNULL(airline.codeIata), airline.codeIata', 'ASC')
  </code></pre>
  
  <p>but this gives me a syntax error.</p>
  
  <p>The Doctrine docs give me no answer either. Is there a way?</p>