SQL Server 查询语句提取中文内容

create   function   fun_getCN(@str   nvarchar(4000))   
  returns   nvarchar(4000)   
  as   
  begin   
  declare   @word   nchar(1),@CN   nvarchar(4000)   
  set   @CN=''   
  while   len(@str)>0   
  begin   
  set   @word=left(@str,1)   
  if unicode(@word)   between   19968   and   19968+20901 
      set   @CN=@CN+@word
  set   @str=right(@str,len(@str)-1)   
  end   
  return   @CN   
  end   

select dbo.fun_getCN('ASDKG论坛KDL')
论坛
select dbo.fun_getCN('ASDKG論壇KDL')
論壇
select dbo.fun_getCN('ASDKDL')
空字符串