获取分组中的最值（group by order by ）

使用子查询。
limit：防止优化器对sql进行优化，导致分组排序无效

SELECT
    * 
FROM
    (
    SELECT
        e.event_id,
        e.user_id,
        e.user_name,
        e.createtime 
    FROM
        event_log e 
    WHERE
        e.type = 10 
    ORDER BY
        e.createtime DESC 
        LIMIT 100000 
    ) t 
GROUP BY
    event_id