数据库版本 mysql 5.7.21
执行语句：
SELECT *,count( id ) AS id
FROM
( SELECT * FROM message ORDER BY created_date DESC) tt
GROUP BY
conversation_id
ORDER BY
created_date DESC;
发现组内排序无效。
但如果在子查询中加入 limit 就能实现组内排序
SELECT *,count( id ) AS id
FROM
( SELECT * FROM message ORDER BY created_date DESC LIMIT 0, 100 ) tt
GROUP BY
conversation_id
ORDER BY
created_date DESC;
原因：5.7以后对排序的sql解析做了优化，子查询中的排序
