WITH t AS
(
SELECT '책' category, '단편소설' category2, '저자' category3, '홍길동,임꺽정,어우동,아무개' category4 FROM dual
UNION ALL 
SELECT '책', '단편소설', '출판사', '동아출판사,해금,빛나' FROM dual
UNION ALL 
SELECT '책', '단편소설', '옮긴이', '이순신,김유신,강감찬,세종대왕' FROM dual
UNION ALL 
SELECT '책', '단편소설', '차례', '새로운시작, 중간, 끝, 등등' FROM dual
UNION ALL 
SELECT '책', '추리소설', '저자', '비, 김태희, 유재석, 김종국' FROM dual
UNION ALL 
SELECT '책', '추리소설', '출판사', '한국출판사, 지구촌, 민음사' FROM dual
UNION ALL 
SELECT '책', '추리소설', '옮긴이', '신사임당, 이이, 이황' FROM dual
UNION ALL 
SELECT '책', '추리소설', '차례', '처음 ,중간, 끝' FROM dual
UNION ALL 
SELECT '책', '추리소설', '기타', '참고사항, 표지크기, 책크기 등등' FROM dual
)
SELECT
    (CASE WHEN category_rnum = 1 THEN category ELSE '' END) AS category
    , (CASE WHEN category_rnum2 = 1 THEN category2 ELSE '' END) AS category2
    , category3
    , category4
FROM
    (
    SELECT
        category
        , category2
        , category3
        , category4
        , ROW_NUMBER() OVER (PARTITION BY category ORDER BY 1) AS category_rnum
        , ROW_NUMBER() OVER (PARTITION BY category, category2 ORDER BY 2) AS category_rnum2
    FROM t
    )
;

WM_CONCAT 은 비공식 함수 입니다.
11G 이상이라면? LISTAGG 를 사용하세요.
http://gurubee.net/article/55512

SELECT DECODE(ROW_NUMBER() OVER(PARTITION BY c1 ORDER BY c2, c3), 1, c1) c1
     , DECODE(ROW_NUMBER() OVER(PARTITION BY c1, c2 ORDER BY c3), 1, c2) c2
     , c3
     , LISTAGG(c4, ',') WITHIN GROUP(ORDER BY 1) c4
  FROM t
 GROUP BY c1, c2, c3
;