mysql rand（）限制错误

Question

您好我有这样的查询：

$query = "select url from music where privacy=0 and musicid in (select musicid from tag where tagname in (select tagname from tag where musicid in (select musicid from music_like where userid=:userid  )) ) ***order by rand() limit 9,***" . $this->musicPerPage;

我的问题是：

我知道这个查询返回20条记录

-- if write ***order by rand() limit 9*** -> no result

-- if write ***order by rand() limit 9,*** -> return 11 record

-- if write ***order by rand() limit 10,*** -> return 10 record

-- if write ***order by rand() limit 11,*** -> return 9 record

有人可以帮助我！