order by打乱了分组select news_title,news_class,news_content,addtime,htm_name from news_content where addtime = (select max(addtime) from new_content group by news_class) group by news_class
select news_title,news_class,news_content,addtime,htm_name from news_content as a where exists (select news_class,max(addtime) as addtime from news_content as b group by news_class where a.news_class = b.news_class and a.addtime = b.addtime)
from news_content as a
where exists
(select news_class,max(addtime) as addtime
from news_content as b
group by news_class where a.news_class = b.news_class and a.addtime = b.addtime)