求简单高效的SQL语句！该怎么处理

求简单高效的SQL语句！！！

SQL code

有表tb(sCode varchar(20),sDt datetime,Station varchar(10)) 给出以下数据22313234133121108500    2011-12-26 14:11:39    互感器22313234133121108500    2011-12-26 14:11:08    互感器31222222244121108548    2011-12-26 14:11:46    互感器31222222244121108548    2011-12-26 14:11:27    互感器31222222244121108548    2011-12-26 12:37:01    互感器31222222244121108548    2011-12-26 10:02:46    互感器22313234133121108500    2011-12-26 13:53:32    接地刀22313234133121108500    2011-12-26 13:51:55    接地刀22313234133121108500    2011-12-26 13:51:39    接地刀31222222244121108548    2011-12-26 13:48:21    接地刀31222222244121108548    2011-12-26 13:48:14    接地刀31222222244121108548    2011-12-26 13:41:59    接地刀需要得出以下数据：22313234133121108500    2011-12-26 14:11:39    互感器31222222244121108548    2011-12-26 14:11:46    互感器22313234133121108500    2011-12-26 13:53:32    接地刀31222222244121108548    2011-12-26 13:48:21    接地刀

SQL code

select * from tb a where not exists(select 1 from tb       where sCode=a.sCode and Station=a.Station and sDt>a.sDt)