mysql多表查询及其 group by 组内排序

//多表查询：得到最新的数据后再执行多表查询

SELECT *FROM `students` `st` RIGHT JOIN(
  SELECT * FROM 　　(　　　　SELECT * FROM goutong WHERE goutongs=‘asdf‘ ORDER BY time DESC　　) AS gtt GROUP BY gtt.name_id ORDER BY gtt.goutong_time DESC ) gt 　　ON `gt`.`name_id`=`st`.`id` LIMIT 10

//先按时间排序查询，然后分组(GROUP BY ) SELECT * FROM
　　(
　　　　SELECT * FROM goutong WHERE goutongs=‘asdf‘ ORDER BY time DESC　　) AS gtt GROUP BY gtt.name_id ORDER BY gtt.time DESC

参考：http://blog.csdn.net/shellching/article/details/8292338

有数据表 comments
------------------------------------------------
| id | newsID | comment |
theTime |
------------------------------------------------
| 1  |        1      |         aaa    |     11       |
------------------------------------------------
| 2  |        1      |         bbb    |     12       |
------------------------------------------------
| 3  |        2      |         ccc     |     12       |

------------------------------------------------

newsID是新闻ID，每条新闻有多条评论comment，theTime是发表评论的时间

现在想要查看每条新闻的最新一条评论：

select * from comments group by newsID 显然不行

select * from comments group by newsID order by theTime desc
是组外排序，也不行

下面有两种方法可以实现：

（1）
selet tt.id,tt.newsID,tt.comment,tt.theTime from(  
select id,newsID,comment,theTime from comments order by theTime desc) as tt group by newsID 

（2）
select id,newsID,comment,theTime from comments as tt group by id,newsID,comment,theTime having
 theTime=(select max(theTime) from comments where newsID=tt.newsID)