ORACLE 树形结构数据 查询某结点下全部子节点无限递归的前2个数据

SELECT *                 FROM (        SELECT     *    FROM TABLE_NAME    CONNECT BY PRIOR _LEVEL_ID = _UPPER_LEVEL_ID    AND _NUM <= 2    START WITH _UPPER_LEVEL_ID IN       (        SELECT         _LEVEL_ID        FROM TABLE_NAME        WHERE _UPPER_LEVEL_ID = 'xxx'         AND _NUM > 2    )     AND _NUM <= 2)WHERE _NUM <= 2  ORDER BY _UPPER_LEVEL_ID;已知某结点 查询其下全部子结点无限递归的前2个数据