group by 查询指定不重复的列
要查询同一个人在不同时间段访问一个空间，读出不重复的最新3个值
select distinct u.username,u.imageurl,a.accesstime from user u join access a on u.userid=a.come and current="+visitUserId+" group by a.come order by a.accesstime desc limit 0,3" ;