Writing a subquery using Zend DB?

Answer

$select = $db->select()->from(array("s" => "sles"), array("s.id","i.id as instanceid","i.reference","i.name","i.sic_code","i.start_date"))
                                 ->join(array('i' => "sle_instances"),"s.id = i.sle_id",array())
                                 ->where("i.id = (select max(id) from sle_instances where sle_id = s.id)")
                                 ->order('i.name asc');