select * from
                             (select 
                             ROW_NUMBER() over(order by [pictures].[id] desc)as rownum,
                            
                             --这个无效distinct [pictures].[id],
                             [pictures].[u_id],
                             [pictures].[title],
                             [pictures].[path],
                             [pictures].[postdatetime]
                             from [pictures]
                             join [picturestags] on [picturestags].[p_id]=[pictures].[id]
                             join [tags] on [tags].[id]=[picturestags].[t_id]
                             where [tags].[name] like @query)as pictures
                             Order by [postdatetime] desc

--try
;with f as
(
select * from
                             (select 
                             ROW_NUMBER() over(order by [pictures].[id] desc)as rownum,
                            
                             --这个无效distinct [pictures].[id],
                             [pictures].[u_id],
                             [pictures].[title],
                             [pictures].[path],
                             [pictures].[postdatetime]
                             from [pictures]
                             join [picturestags] on [picturestags].[p_id]=[pictures].[id]
                             join [tags] on [tags].[id]=[picturestags].[t_id]
                             where [tags].[name] like @query)as pictures
)

select * from f t where [rownum]=(select max([rownum]) from f where postdatetime=t.postdatetime)

------解决方案--------------------
group by

------解决方案--------------------

SQL code

 ;with aaa as
 (select 
 ROW_NUMBER() over(order by [pictures].[id] desc)as rownum,
 [pictures].[u_id],
 [pictures].[title],
 [pictures].[path],
 [pictures].[postdatetime]
 from [pictures]
 join [picturestags] on [picturestags].[p_id]=[pictures].[id]
 join [tags] on [tags].[id]=[picturestags].[t_id]
 where [tags].[name] like @query)
 select * from aaa as a where not exists (select 1 from aaa 
 where aaa.u_id=a.u_id and aaa.title=a.title and aaa.[path]=a.[path] and aaa.postdatetime=a.postdatetime and aaa.rownum<a.rownum)