预期结束字符串，得到'IS'

Question

Symfony抛出异常查询解析： 错误：字符串的预期结束，得到'IS'

$query = $em->createQuery('
    SELECT a,
    s2.id AS id_service,
    s2.priority AS priority,
    m.name AS map_name, m.id AS id_map
  FROM GameServerBundle:Server\Server a
    LEFT JOIN a.services ss
    LEFT JOIN ss.service s2
    LEFT JOIN a.map m
    LEFT JOIN a.modification mod
  WHERE a.isBanned = 0
  ORDER BY ss.serviceId IS EMPTY, priority DESC
');