DELETE FROM 테이블명
WHERE 식별값 not in ( SELECT 식별값 from ( SELECT 식별값 from 테이블명 group by 중복되는 칼럼) as 식별값 )
예를 들어
DELETE FROM ask_match_category
WHERE idx not in ( SELECT idx from ( SELECT idx from ask_match_category group by cin) as idx )
테이블명 : ask_match_category
식별값(고유값) : idx
중복되는 칼럼 : cin
으로 바꿔서 사용함