title: DQL LEFT JOIN - sql example
body: <p>well-functioning SQL is:</p>
  
  <pre><code>SELECT ro.id_role
         ,rr.id_role_resource
         ,re.id_resource
      FROM resource re
      LEFT JOIN role_resource rr
          ON rr.resource_id = re.id_resource
      LEFT JOIN role ro
          ON ro.id_role = rr.role_id
  </code></pre>
  
  <p>now I must write DQL-
  this is table shema:</p>
  
  <p>role: <code>id_role, name</code></p>
  
  <p>role_resource: <code>id_role_resource, role_id, resource_id, permission</code></p>
  
  <p>resource: <code>id_resource, controller,action ...</code></p>
  
  <p>in the last table are no corresponding records in the table role_resource.
  That's why i need this left Join Query in DQL.</p>