在没有循环的情况下插入或更新?

无需循环即可完成是:

UPDATE table1
SET Quantity = Quantity + 1
WHERE Item = @itemID

IF @@ROWCOUNT = 0
    INSERT INTO table1 (Item, Quantity)
    VALUES (@itemID, 1)

 IF EXISTS (SELECT * from ItemMaster WHERE Item = @Item)
    UPDATE ItemMaster
      SET Quantity = @Quantity
    WHERE Item = @Item
 ELSE
    INSERT INTO ItemMaster VALUES(@Item, @Quantity)

对于多行:

 INSERT INTO ItemMaster (Item, Quantity)
 SELECT Item, Quantity
 FROM AnotherTable a
 WHERE NOT EXISTS (Select 1 from ItemMaster i Where i.Item = a.Item);

 UPDATE ItemMaster i
    SET Quantity = a.Quantity
 FROM AnotherTable a
 WHERE a.Item = i.Item