在 SQL 中计算派生属性的值

Question

我有一个包含两张桌子的餐厅数据库。其中一张表是'item'，具有以下属性：

ItemID, ItemName, Price and Cuisine.

我有另一个表'bill'，它具有以下属性：

OrderID, Item1_ID, Item2_ID, Item3_ID, Quantity_1, Quantity_2, Quantity_3, Total.

bill 的第 2、3、4 属性指定了 3 个具有不同 ItemID 的商品，第 5、6、7 属性指定了它们各自的数量。我想通过将它们各自的数量乘以它们的价格（取自“项目”表）来计算每个项目的价格，然后将它们各自的价格相加并保存结果。

1.我能够生成三个单独的价格，但由于它们是派生属性，我无法保存它们并因此更新“账单”的“总计”属性。我该怎么做呢？这是我用来生成三个单独价格的代码：

SELECT A.Total_1, B.Total_2, C.Total_3
FROM
(SELECT bill.orderid, bill.quantity_1 * item.Price AS 'Total_1'
FROM bill, item 
WHERE item.itemid = bill.item_1
)A,

(SELECT bill.orderid, bill.quantity_2 * item.Price AS 'Total_2'
FROM bill, item 
WHERE item.itemid = bill.item_2
)B,

(SELECT bill.orderid, bill.quantity_3 * item.Price AS 'Total_3'
FROM bill, item 
WHERE item.itemid = bill.item_3
)C

WHERE A.orderid = B.orderid AND A.orderid = C.orderid;

1. 我应该删除属性“总计”，因为它是派生的？我确实希望在另一个表中使用它的值，那么他们有什么其他方法可以解决这个问题吗？

Answer 1

你可以像这样使用 somting：

select 
    item_1 * price_1 + item_2 * price_2 + item_3 * price_3 as total
    item_1 * price_1 as total_1
    item_2 * price_2 as total_2
    item_3 * price_3 as total_3
where orderId = yourOrderId

如果您在选择中进行逻辑操作（+、*、/、-），则与您将项目分开并以您喜欢的语言进行操作一样快。

Answer 2

一种快速简便的解决方案是将现有查询用作派生表并从中进行选择：

SELECT Total_1, Total_2, Total_3, 
 Total_1 + Total_2 + Total_3 AS GrandTotal
FROM (
SELECT A.Total_1, B.Total_2, C.Total_3
FROM
(SELECT bill.orderid, bill.quantity_1 * item.Price AS 'Total_1'
FROM bill, item 
WHERE item.itemid = bill.item_1
)A,

(SELECT bill.orderid, bill.quantity_2 * item.Price AS 'Total_2'
FROM bill, item 
WHERE item.itemid = bill.item_2
)B,

(SELECT bill.orderid, bill.quantity_3 * item.Price AS 'Total_3'
FROM bill, item 
WHERE item.itemid = bill.item_3
)C

WHERE A.orderid = B.orderid AND A.orderid = C.orderid
) T;

编辑：

要将GrandTotal 存储在表Bill 的Total 列中，您可以这样做：

UPDATE b2
SET Total = (
 SELECT A.Total_1 + B.Total_2 + C.Total_3
    FROM
    (SELECT bill.quantity_1 * item.Price AS 'Total_1'
    FROM bill, item 
    WHERE item.itemid = bill.item_1
    AND bill.Orderid=b2.Orderid
    )A,

    (SELECT bill.quantity_2 * item.Price AS 'Total_2'
    FROM bill, item 
    WHERE item.itemid = bill.item_2
    AND bill.Orderid=b2.Orderid
    )B,

    (SELECT bill.quantity_3 * item.Price AS 'Total_3'
    FROM bill, item 
    WHERE item.itemid = bill.item_3
    AND bill.Orderid=b2.Orderid
    )C
)
FROM Bill b2