MYSQL重复记录排除法处理方式

SELECT
tmp.user_id,
tmp.course_id,
tmp.type,
tmp.expire_time,
@rownum := @rownum + 1,

IF (
@course_id = tmp.course_id ,@rank := @rank + 1,
@rank := 1
) AS rank,
@course_id := tmp.course_id
FROM
(
SELECT
user_id,
course_id,
type,
add_time,
expire_time
FROM
xz_user_course
WHERE
user_id = 296154
ORDER BY
course_id ASC,
expire_time DESC
) tmp,
(
SELECT
@rownum := 0,
@course_id := 0,
@rank := 0
) a