select * 
from tb as a 
where not exists(select 1 from tb username=a.username and chengji>a.chengji)

------解决方案--------------------
SQL code
--处理表重复记录(查询和删除) 
 /****************************************************************************************************************************************************** 
 1、Num、Name相同的重复值记录,没有大小关系只保留一条 
 2、Name相同,ID有大小关系时,保留大或小其中一个记录 
 整理人：中国风(Roy) 
  
 日期:2008.06.06 
 ******************************************************************************************************************************************************/ 
  
 --1、用于查询重复处理记录(如果列没有大小关系时2000用生成自增列和临时表处理,SQL2005用row_number函数处理) 
  
 --> --> (Roy)生成測試數據 
   
 if not object_id('Tempdb..#T') is null 
   drop table #T 
 Go 
 Create table #T([ID] int,[Name] nvarchar(1),[Memo] nvarchar(2)) 
 Insert #T 
 select 1,N'A',N'A1' union all 
 select 2,N'A',N'A2' union all 
 select 3,N'A',N'A3' union all 
 select 4,N'B',N'B1' union all 
 select 5,N'B',N'B2' 
 Go 
  
  
 --I、Name相同ID最小的记录(推荐用1,2,3),方法3在SQl05时，效率高于1、2 
 方法1: 
 Select * from #T a where not exists(select 1 from #T where Name=a.Name and ID <a.ID) 
  
 方法2: 
 select a.* from #T a join (select min(ID)ID,Name from #T group by Name) b on a.Name=b.Name and a.ID=b.ID 
  
 方法3: 
 select * from #T a where ID=(select min(ID) from #T where Name=a.Name) 
  
 方法4: 
 select a.* from #T a join #T b on a.Name=b.Name and a.ID>=b.ID group by a.ID,a.Name,a.Memo having count(1)=1  
  
 方法5: 
 select * from #T a group by ID,Name,Memo having ID=(select min(ID)from #T where Name=a.Name) 
  
 方法6: 
 select * from #T a where (select count(1) from #T where Name=a.Name and ID <a.ID)=0 
  
 方法7: 
 select * from #T a where ID=(select top 1 ID from #T where Name=a.name order by ID) 
  
 方法8: 
 select * from #T a where ID!>all(select ID from #T where Name=a.Name) 
  
 方法9(注:ID为唯一时可用): 
 select * from #T a where ID in(select min(ID) from #T group by Name) 
  
 --SQL2005: 
  
 方法10: 
 select ID,Name,Memo from (select *,min(ID)over(partition by Name) as MinID from #T a)T where ID=MinID 
  
 方法11: 
  
 select ID,Name,Memo from (select *,row_number()over(partition by Name order by ID) as MinID from #T a)T where MinID=1 
  
 生成结果: 
 /* 
 ID      Name Memo 
 ----------- ---- ---- 
 1       A  A1 
 4       B  B1 
  
 (2 行受影响) 
 */ 
  
  
 --II、Name相同ID最大的记录,与min相反: 
 方法1: 
 Select * from #T a where not exists(select 1 from #T where Name=a.Name and ID>a.ID) 
  
 方法2: 
 select a.* from #T a join (select max(ID)ID,Name from #T group by Name) b on a.Name