SQL UPDATE FROM 的使用方法

http://www.frogjumpjump.com/2011/09/sql-update-from.html

UPDATE Product
SET ProductCategoryCode = o.ProductCategory
FROM OldProduct o JOIN Product p
ON o.SerialNumber = p.SerialNumber

http://social.msdn.microsoft.com/Forums/zh-TW/ec192db7-d8f9-42f4-b179-adbfdf90e5b2/sql?forum=238

select a.*,b.amount
from view_backpackinfo a
inner join (
  SELECT  item_id,username,SUM(amount) as amount
   FROM view_backpackinfo  
   group by item_id,username
) b on a.item_id = b.item_id and a.username = b.username