Question

我正在研究一个简单的MySQL订购系统，我遇到了这个障碍，我希望一些SQL天才可以帮助我。

我有一个表，用于订单，付款（带有对Order表的外键引用）和OrderItems（也有一个对Order表的外键引用），我想要做的是获得总计未完成的使用单个查询的订单余额（总计和付费）。我最初的想法是做一些像这样简单的事情：

SELECT Order.*, SUM(OrderItem.Amount) AS Total, SUM(Payment.Amount) AS Paid
FROM Order
JOIN OrderItem ON OrderItem.OrderId = Order.OrderId
JOIN Payment ON Payment.OrderId = Order.OrderId
GROUP BY Order.OrderId

但是，如果有多个付款或多个OrderItems，它会分别混淆Total或Paid（例如，一个OrderItem记录，金额为100，另外两个付款记录将产生总计200）。

为了克服这个问题，我可以通过以下方式使用一些子查询：

SELECT Order.OrderId, OrderItemGrouped.Total, PaymentGrouped.Paid
FROM Order
JOIN (
    SELECT OrderItem.OrderId, SUM(OrderItem.Amount) AS Total
    FROM OrderItem
    GROUP BY OrderItem.OrderId
) OrderItemGrouped ON OrderItemGrouped.OrderId = Order.OrderId
JOIN (
    SELECT Payment.OrderId, SUM(Payment.Amount) AS Paid
    FROM Payment
    GROUP BY Payment.OrderId
) PaymentGrouped ON PaymentGrouped.OrderId = Order.OrderId

你可以想象（并且这个查询会显示EXPLAIN），这不是一个最佳查询，所以，我想知道，有没有办法用{{1转换这两个子查询}}语句到GROUP BY s？

Answer 1

使用正确的索引可能会更快：

select o.OrderId,
       (select sum(oi.Amount)
        from OrderItem oi
        where oi.OrderId = o.OrderId
       ) as Total,
       (select sum(p.Amount)
        from Payment p
        where oi.OrderId = o.OrderId
       ) as Paid
from Order o;

正确的索引是OrderItem(OrderId, Amount)和Payment(OrderId, Amount)。

我不喜欢用这种方式编写聚合查询，但它有时可以帮助MySQL中的性能。

Answer 2

有些答案已经建议使用相关的子查询，但是没有真正解释为什么。 MySQL没有实现相关的子查询，但它将实现派生表。也就是说现在使用查询的简化版本：

SELECT Order.OrderId, OrderItemGrouped.Total
FROM Order
JOIN (
    SELECT OrderItem.OrderId, SUM(OrderItem.Amount) AS Total
    FROM OrderItem
    GROUP BY OrderItem.OrderId
) OrderItemGrouped ON OrderItemGrouped.OrderId = Order.OrderId;

在执行开始时，MySQL会将子查询的结果放入临时表中，并在OrderId上散列此表以便更快地查找，而如果运行：

SELECT  Order.OrderId,
        (   SELECT SUM(OrderItem.Amount)
            FROM OrderItem
            WHERE OrderItem.OrderId = OrderId
        ) AS Total
FROM    Order;

对于Order中的每一行，子查询将被执行一次。如果添加WHERE Order.OrderId = 1之类的东西，聚合整个OrderItem表显然效率不高，将结果散列为仅查找一个值，但如果要返回所有订单，则创建哈希表的初始成本将使为了它自己，它不必为Order表中的每一行执行子查询。

如果您选择了很多行并认为实现有益，您可以按如下方式简化您的JOIN查询：

SELECT  Order.OrderId, SUM(OrderItem.Amount) AS Total, PaymentGrouped.Paid
FROM    Order
        INNER JOIN OrderItem
            ON OrderItem.OrderID = Order.OrderID
        INNER JOIN 
        (   SELECT Payment.OrderId, SUM(Payment.Amount) AS Paid
            FROM Payment
            GROUP BY Payment.OrderId
        ) PaymentGrouped 
            ON PaymentGrouped.OrderId = Order.OrderId;
GROUP BY Order.OrderId, PaymentGrouped.Paid;

然后你只有一个派生表。

Answer 3

这样的事情：

SELECT Order.OrderId, (
SELECT SUM(OrderItem.Amount) 
FROM OrderItem as OrderItemGrouped
where
OrderItemGrouped.OrderId = Order.OrderId
), AS Total,
(
SELECT SUM(Payment.Amount) 
FROM Payment as PaymentGrouped
where
PaymentGrouped.OrderId = Order.OrderId
) as Paid
FROM Order

PS：你再次获胜@Gordon xD

Answer 4

Select o.orderid, i.total, s.paid
From orders o
Left join (select orderid, sum(amount)
From orderitem) i
On i.orderid = o.orderid
Ieft join (select orderid, sum(amount)
From payments) s
On s.orderid = o.orderid

使用GROUP BY将多个子查询转换为JOIN

4 个答案: