IFNULL在Symfony2学说查询生成器中如何实现SQL？假设我有这个查询：

select * from ticket order by IFNULL(modified_date, '2000-01-01') DESC, created_date DESC

我有这个DQL：

$this->qb->select("t, c.name")
         ->from("Ticket", "t");
$this->qb->orderBy("t.modifiedDate", "DESC");
$this->qb->addOrderBy("t.createdDate", "DESC");

现在如何添加IFNULL部分？