sql根据某一个字段重复只取第一条数据

SELECT * FROM(  
    SELECT V.VENDOR_CODE,
           V.VENDOR_NAME,
           V.VENDOR_TYPE,
           V.SMTP_ADDR,
           V.MOB_NUMBER,
           U.USER_NAME,
           ROW_NUMBER() OVER(ORDER BY V.VENDOR_CODE) AS RN
    FROM T_BD_VENDOR V,
         T_BASE_USER U,
         '(SELECT S.*   
            FROM (  
                 SELECT T.*, ROW_NUMBER() OVER (PARTITION BY T.VENDOR_CODE ORDER BY T.USER_CODE) AS GROUP_IDX   
                 FROM T_BD_USER_VENDOR_REL T 
                 )S 
            WHERE S.GROUP_IDX = 1
         )R'
    WHERE 1=1
          AND V.VENDOR_CODE = R.VENDOR_CODE(+)
          AND R.USER_CODE = U.USER_CODE(+)
          )
  WHERE RN BETWEEN 1 AND 20

其中绿色部分表示从中间关联表中根据VENDOR_CODE字段重复的只取出一条。 如上图只取出根据VENDOR_CODE分组，根据USER_CODE排序的第一条数据。