-- 8i --
SELECT num
     , MIN(DECODE(rn,1,''''||data||''''))
    || MIN(DECODE(rn,2,','''||data||''''))
    || MIN(DECODE(rn,3,','''||data||''''))
  FROM (SELECT num, data
             , ROW_NUMBER() OVER(PARTITION BY num ORDER BY data) rn
          FROM t
        )
 GROUP BY num
;

-- 9i --
SELECT num
     , SUBSTR(MAX(SYS_CONNECT_BY_PATH(''''||data||'''',',')),2) data
  FROM (SELECT num, data
             , ROW_NUMBER() OVER(PARTITION BY num ORDER BY data) rn
          FROM t
        )
 START WITH rn = 1
 CONNECT BY PRIOR num = num
        AND PRIOR rn = rn - 1
 GROUP BY num
;

-- 10G --
SELECT num
     , REPLACE(SUBSTR(data,2),'@','''') data
  FROM
(
SELECT num,
XMLAGG(XMLELEMENT(x,',@'||data||'@') ORDER BY data).EXTRACT('//text()').GetStringVal() data
  FROM t
 GROUP BY num
)
;

10G XML 함수에서는 따옴표가 ' 로 표시되는 관계로
부득이하게 @ 로 대체해서 다시한번 처리했습니다.