예시자료의 현재 출력 순서를 보장 할 수 있는 항목이 필요합니다.
해당 항목이 추가된 예시자료가 있어야 합니다.
순번이라던가? 등록일시 라던가? 하는 정렬 기준 항목.

DENSE_RANK() OVER(PARTITION BY XXX ORDER BY ZZZ) AS 순번

WITH t AS
(
-- 정렬용 항목(seq) 추가
SELECT 'A' code, 1 seq, 'AAA' code_nm FROM dual
UNION ALL SELECT 'A', 2, 'AAA' FROM dual
UNION ALL SELECT 'A', 3, 'BBB' FROM dual
UNION ALL SELECT 'A', 4, 'BBB' FROM dual
UNION ALL SELECT 'A', 5, 'AAA' FROM dual
UNION ALL SELECT 'A', 6, 'AAA' FROM dual
UNION ALL SELECT 'B', 1, 'CCC' FROM dual
UNION ALL SELECT 'B', 2, 'CCC' FROM dual
UNION ALL SELECT 'B', 3, 'DDD' FROM dual
UNION ALL SELECT 'B', 4, 'DDD' FROM dual
UNION ALL SELECT 'B', 5, 'CCC' FROM dual
UNION ALL SELECT 'B', 6, 'CCC' FROM dual
)
SELECT code
     , seq
     , code_nm
     , SUM(flag) OVER(PARTITION BY code ORDER BY seq) grp
  FROM (SELECT code
             , seq
             , code_nm
             , DECODE( LAG(code_nm) OVER(PARTITION BY code ORDER BY seq)
                     , code_nm, 0, 1) flag
          FROM t
        )
;