oracle重复字段设置为空函数

SELECT CASE
         WHEN PRE_NAME IS NULL THEN
          USER_NAME
         ELSE
          NULL
       END AS USER_NAME,
       COURSE,
       GRADE
  FROM (SELECT A.*,
               LAG(USER_NAME) OVER(PARTITION BY USER_NAME ORDER BY COURSE) AS PRE_NAME
          FROM TEST_XX A)