關於sql中字串的疑惑

今天早上寫了一個預存程序,如下

DECLARE packway_cursor CURSOR FOR
select Distinct  by00902 from sdby009 where by00901 in(
select  by02108 from sdby021 where by02118=@by02118)
OPEN packway_cursor
FETCH NEXT FROM packway_cursor
INTO @PackWay
while(@@fetch_status=0)    
begin 
   set @PackMeanId =@PackMeanId + @PackWay
  fetch next from packway_cursor into @PackWay
  if(@@fetch_status=0)
  begin
   set @PackMeanId = @PackMeanId + '/'
  end
end

close packway_cursor      
deallocate packway_cursor
select @PackMeanId

問題是無論有沒有記錄存在@PackMeanId 都為null,不知道原因是什麼,而我將他改為如下

DECLARE packway_cursor CURSOR FOR
select Distinct  by00902 from sdby009 where by00901 in(
select  by02108 from sdby021 where by02118=@by02118)
OPEN packway_cursor
FETCH NEXT FROM packway_cursor
INTO @PackWay
set @PackMeanId = ''
while(@@fetch_status=0)    
begin 
   set @PackMeanId =@PackMeanId + @PackWay
  fetch next from packway_cursor into @PackWay
  if(@@fetch_status=0)
  begin
   set @PackMeanId = @PackMeanId + '/'
  end
end

close packway_cursor      
deallocate packway_cursor
select @PackMeanId

紅色的為後來加的,這樣就能得到正確的答案的,原因何在呢,難道是定義的@PackMeanId,沒有給他賦值,他為null,然後 set @PackMeanId =@PackMeanId + @PackWay就一直為null嗎?不明?