有两张表a,b

有两张表bookTable   commentTable
id(identity 1,1,primaty key)   bookname(varchar(50)
1                                   xml
2                                   c++
3                                   sql
4                                    oracal
b:
id(identity 1,1,primary key)      bookId(a的外键)  userId      content(text)      commenttime(DateTime)  
1                                     1              d            dfgjkkghh          2006-9-29
2                                     1              d           jkkl;gkl;          2006-9-29
3                                     2              a           dfdfdhghdk        2006-9-28
4                                     2              a           65676986986       2006-5-9
5                                     3                          ggghghgh          2006-9-10
6                                     4                          ghjkjkl            2006-9-25
7                                     1                           rsggah            2006-9-16

现在要选出根据时间降序排列，排名前三的无重复的记录，其中要包括bookid name  content  time 的信息
这个SQL语句如何写啊？

xml c#           8 333333   2006-09-27      admin
asp.net           4 NULL    NULL             NULL
asp.net 2.0   5 NULL   NULL            NULL


select rowid = identity(int,1,1),a.bookname,a.id,b.userId,b.title,b.content,b.commenttime into #3
from bookTable as a
right join commentTable as b on a.id=b.bookId
order by b.commenttime desc

select TOP 3 bookname,id,content,commenttime,userId
from #3 d
where not exists(select 1 from  #3 where  rowid < d.rowid and d.userId=userid)
drop table #3
select 1 指选出第一笔数据
当 join后选出来的数据有空的null时，如果不要空的fight join 改成left join
这里的select 1 指的是选出count(*)行，每一行都是1，针对的是identity类型
--------------------------------
alter   table   tablename   add   id   int   identity(1,1)  
  go  
  delect   from   tablename   where   id   not   in(select   min(id)   from   tablename   group   by   no,name)  
  go  
  alter   table   tablename   drop   column   id


select rowid = identity(int,1,1),a.name,a.bookid,b.content,b.time into #
from a as a
left join b as b on a.bookid=b.bookId
order by b.time desc

select TOP 3 name,bookid,content,time
from # d
where not exists(select 1 from  # where d.name = name and rowid < d.rowid )
drop table #