查询树型表函数的问题，崩溃啊

CREATE function f_pid1(@id int,@tablename varchar(30)) returns varchar(100)

as

begin

  declare @re_str as varchar(100)

  set @re_str = ''

  select @re_str = name from @tablename where id = @id

  while exists (select 1 from @tablename where id = @id and pid is not null)

    begin

      select @id = b.id , @re_str = b.name + ',' + @re_str from @tablename a , @tablename b where a.id = @id and a.pid = b.id

    end

  return @re_str

end

他说我没声明@tablename,这个是传过来的参数啊，郁闷，到底怎么改，也有网友说用exec动态执行，但是我改来改去还是有bug，请教啊

CREATE function f_pid1(@id int,@tablename varchar(20)) returns varchar(100)

as

begin

  declare @re_str as varchar(100)

  set @re_str = ''

  exec('select '+@re_str+' = name from '+@tablename+' where id = '+rtrim(@id))

  exec('while exists (select 1 from tb where id = '+rtrim(@id)+' and pid is not null)

    begin

      select '+rtrim(@id)+' = b.id , '+@re_str+' = b.name + '','' + '+@re_str+' from '+@tablename+' a , tb b where a.id = '+rtrim(@id)+' and a.pid = b.id

    end')

  return @re_str

end