取出分组后的前N条数据，笔记记录。

sql2005及其以后可以用，使用cte来，

WITH ct

     AS (SELECT at.peoplemark,

                at.TraceTitle,

                at.traceid,

                at.PostDate

         FROM   cms_article_traceback at,

                cms_people_extension p

         WHERE  p.articleid = at.PeopleId

                AND p.classid = 438)

SELECT TOP 20 peoplemark,

              TraceTitle,

              traceid,

              PostDate

FROM   (SELECT peoplemark,

               TraceTitle,

               traceid,

               PostDate,

               Row_number()

                 OVER (

                   partition BY traceid

                   ORDER BY postdate DESC) rn

        FROM   ct) b

WHERE  b.rn = 1

ORDER  BY b.postdate DESC 

如果在sql2000，那

select * from tb t where not exists(select 1 from where userid=t.userid and [time]>t.[time])