select row_number() over(partion by mntr_seq order by user_nm) as rn

---> 순차적 번호를 부여하되, mntr_seq별로 나눠 각 분류를 user_nm의 오름차순으로 정렬한 결과에 순차 번호를 부여한다.

rn                mntr_seq              user_nm

---------------------------------------

1                    123                    김개똥

1                    124                    홍길동

1                    125                    장보고

1                    126                    김개똥

2                    126                    김삿갓

3                    126                    장보고

1                    127                    홍길동

[출처] over(partition by COL_NM order by COL_NM)|작성자 realuv