WITH T AS
(
SELECT 'M' ID  ,NULL P_ID  ,0 LV  ,'TOP' NM      FROM DUAL UNION ALL
SELECT 'M1'     ,'M'        ,1     ,'소프트웨어'  FROM DUAL UNION ALL
SELECT 'M11'    ,'M1'       ,2     ,'웹'          FROM DUAL UNION ALL
SELECT 'M111'   ,'M11'      ,3     ,'php'         FROM DUAL UNION ALL
SELECT 'M112'   ,'M11'      ,3     ,'jsp'         FROM DUAL UNION ALL
SELECT 'M113'   ,'M11'      ,3     ,'html'        FROM DUAL UNION ALL
SELECT 'M12'    ,'M1'       ,2     ,'운영체제'    FROM DUAL UNION ALL
SELECT 'M13'    ,'M1'       ,2     ,'윈도우어플리케이션'  FROM DUAL UNION ALL
SELECT 'M2'     ,'M'        ,1     ,'하드웨어'    FROM DUAL
)
SELECT MIN(LV),REPLACE(LISTAGG(NM,',') WITHIN GROUP (ORDER BY ID),',(','(') NM 
  FROM
    (
    SELECT  ID, P_ID, LV, NM , ID2
      FROM(
            SELECT MAX(ID) ID, MAX(P_ID) P_ID, MAX(LV) LV
                 , DECODE(MAX(LV),3,'(','')||LISTAGG(NM,',') WITHIN GROUP(ORDER BY P_ID)||DECODE(MAX(LV),3,')','') NM
                 , DECODE(MAX(LV),3,SUBSTR(MAX(ID),1,3),MAX(ID)) ID2
            FROM T
            GROUP BY DECODE(LV,3,LV,ROWNUM-1000)
          )
     START WITH P_ID = 'M'
   CONNECT BY PRIOR ID = P_ID
    )
 GROUP BY ID2

결과값은 나오는데.. 그닥 좋은 방법은 아닌듯...

-- jkson님 WITH문 사용했습니다.

WITH T AS
(
SELECT 'M' ID  ,NULL P_ID  ,0 LV  ,'TOP' NM      FROM DUAL UNION ALL
SELECT 'M1'     ,'M'        ,1     ,'소프트웨어'  FROM DUAL UNION ALL
SELECT 'M11'    ,'M1'       ,2     ,'웹'          FROM DUAL UNION ALL
SELECT 'M111'   ,'M11'      ,3     ,'php'         FROM DUAL UNION ALL
SELECT 'M112'   ,'M11'      ,3     ,'jsp'         FROM DUAL UNION ALL
SELECT 'M113'   ,'M11'      ,3     ,'html'        FROM DUAL UNION ALL
SELECT 'M12'    ,'M1'       ,2     ,'운영체제'    FROM DUAL UNION ALL
SELECT 'M13'    ,'M1'       ,2     ,'윈도우어플리케이션'  FROM DUAL UNION ALL
SELECT 'M2'     ,'M'        ,1     ,'하드웨어'    FROM DUAL
)
SELECT T.LV, T.NM||AA.NM NM
FROM T
   , (SELECT P_ID
           , MAX(LV) LV
           , '('||SUBSTR(XMLAgg(XMLElement(x, ',', NM) ORDER BY ID).Extract('//text()'), 2)||')' NM
      FROM T
      GROUP BY P_ID
      ) AA
WHERE T.LV BETWEEN 1 AND 2
  AND T.ID = AA.P_ID(+)
  AND AA.LV(+) = 3
ORDER BY T.ID
;

-- jkson님 WITH문 사용했습니다.

WITH T AS
(
SELECT 'M' ID  ,NULL P_ID  ,0 LV  ,'TOP' NM      FROM DUAL UNION ALL
SELECT 'M1'     ,'M'        ,1     ,'소프트웨어'  FROM DUAL UNION ALL
SELECT 'M11'    ,'M1'       ,2     ,'웹'          FROM DUAL UNION ALL
SELECT 'M111'   ,'M11'      ,3     ,'php'         FROM DUAL UNION ALL
SELECT 'M112'   ,'M11'      ,3     ,'jsp'         FROM DUAL UNION ALL
SELECT 'M113'   ,'M11'      ,3     ,'html'        FROM DUAL UNION ALL
SELECT 'M12'    ,'M1'       ,2     ,'운영체제'    FROM DUAL UNION ALL
SELECT 'M13'    ,'M1'       ,2     ,'윈도우어플리케이션'  FROM DUAL UNION ALL
SELECT 'M2'     ,'M'        ,1     ,'하드웨어'    FROM DUAL
)
SELECT LV, T.NM||T2.NM
FROM T, (
          select SUBSTR(ID,1,3) ID,'('||LISTAGG(NM,',') WITHIN GROUP (ORDER BY ID)||')' NM
          from t
          where lv = 3
          GROUP BY SUBSTR(ID,1,3)) T2
WHERE T.ID = T2.ID(+)
  AND LV BETWEEN 1 AND 2
ORDER BY T.ID;