정렬기준이 반드시 필요한 문제입니다.
정렬기준 RN 을 추가하여 풀었습니다.
WITH t AS ( SELECT 1 rn, 1 a FROM dual UNION ALL SELECT 2, 1 FROM dual UNION ALL SELECT 3, 2 FROM dual UNION ALL SELECT 4, 3 FROM dual UNION ALL SELECT 5, 3 FROM dual UNION ALL SELECT 6, 1 FROM dual UNION ALL SELECT 7, 2 FROM dual UNION ALL SELECT 8, 3 FROM dual UNION ALL SELECT 9, 3 FROM dual ) SELECT rn, a FROM (SELECT rn, a , DECODE(LAG(a) OVER(ORDER BY rn), a, 0, 1) flag FROM t ) WHERE flag = 1 ;