目测1楼第一句不对，没有保证不同的主题，我是这么写的：

select * from posts p where p.dateline = (select max(dateline) from posts where tid = p.tid) order by dateline desc limit 10;

其实dateline和id应该顺序是一致的，可以考虑换成id，id上主键应该效率更高：

select * from posts p where p.id = (select max(id) from posts where tid = p.tid) order by id desc limit 10;

第2句个人觉得最好还是不用ORDER BY RAND()，数据量稍微多一点就会效率低下，因为这样写可以会导致多次扫描，这样写可能会好一点：

SELECT * FROM posts WHERE id >= ((SELECT MAX(id) FROM posts)-(SELECT MIN(id) FROM posts)) * RAND() + (SELECT MIN(id) FROM posts)  LIMIT 1;

试试看，如果表数据过大不建议使用ORDER BY RAND()，你先试试这两句SQL行不行。希望对你有帮助.

SELET `id`,`tid`,`subject`,`message`,`dateline` FROM `posts` WHERE `tid`=NULL ORDER BY `tid`,`dateline`

SELET `id`,`tid`,`subject`,`message`,`dateline` FROM `posts` WHERE `tid`=!NULL ORDER BY RAND() LIMIT 1

楼上第一条回答目测不对吧，以下个人见解，欢迎点评指教

select t2.* from (select tid,max(dateline) maxDateline from posts group by tid order by maxDateline desc limit 10)t1 inner join posts t2 on t1.tid=t2.tid and t1.maxDateline=t2.dateline