title: use IFNULL in DQL and symfony query
body: <p>subquery and querybuilder in my project 
  but when use ```IFNULL`` command in query return eroor
  my code is bellow </p>
  
  <pre><code>  $subQb = $em-&gt;createQueryBuilder();
              $subquery = $subQb-&gt;select('COUNT(v.id)')
                  -&gt;from('AdminBundle:Visitsite', 'v')
                  -&gt;where('v.site = s.id')
                  -&gt;Andwhere('v.createdate &gt; :date')
                  -&gt;getDQL();
  
              $subQb2 = $em-&gt;createQueryBuilder();
              $subquery2 = $subQb2-&gt;select('l.quantity')
                  -&gt;from('AdminBundle:Limitviewday', 'l')
                  -&gt;where($subQb2-&gt;expr()-&gt;eq('s.limitviewday', 'l.id'))
                  -&gt;getDQL();
  
  
              $subQb3 = $em-&gt;createQueryBuilder();
              $subquery3 = $subQb3-&gt;select('COUNT(i.id)')
                  -&gt;from('AdminBundle:Visitsite', 'i')
                  -&gt;where('i.id = s.id')
                  -&gt;Andwhere('i.createdate &gt; :date2')
                  -&gt;Andwhere('i.ip = :ip')
                  -&gt;groupBy('i.ip')
                  -&gt;getDQL();
  
              $subQb4 = $em-&gt;createQueryBuilder();
              $subquery4 = $subQb4-&gt;select('ipl.quantity')
                  -&gt;from('AdminBundle:Iplimitview', 'ipl')
                  -&gt;where('s.iplimitview = ipl.id')
                  -&gt;getDQL();
  
  
              $qb = $em-&gt;createQueryBuilder();
              $query = $qb-&gt;select('s')
                  -&gt;from('AdminBundle:Sites', 's')
                  -&gt;where('s.quantity &gt; :one')
                  -&gt;Andwhere('s.status = :two')
                  -&gt;Andwhere($qb-&gt;expr()-&gt;lt("($subquery)", "($subquery2)"))
                  -&gt;Andwhere($qb-&gt;expr()-&gt;lt("(SELECT IFNULL(  ($subquery3),0) )", "($subquery4))"))
                  -&gt;setParameter('one', 1)
                  -&gt;setParameter('two', 1)
                  -&gt;setParameter('date', $date-&gt;format('Y-m-d'))
                  -&gt;setParameter('ip', $ip)
                  -&gt;setParameter('date2', $date-&gt;format('Y-m-d 00:00:00'));
  </code></pre>
  
  <p>and my result is </p>
  
  <blockquote>
    <p>[Syntax Error] line 0, col 274: Error: Expected known function, got
    'IFNULL'</p>
  </blockquote>