MySQL刪除重複數據 並保留ID最大值

DELETE
a
FROM
    user1 a
JOIN (
    SELECT
        user_name,
        count(*),
        MAX(id) AS id
    FROM
        user1
    GROUP BY
        user_name,over
    HAVING
        COUNT(*) > 1
) b ON a.user_name = b.user_name
WHERE
    a.id < b.id