如何在MYSQL中减去两个声明的变量

斯图尔特·迪拉德

我正在研究的问题如下：

与2003年相比，2004年每个月收到的金额有何不同？

到目前为止，这就是我所拥有的，

    SELECT @2003 = (SELECT sum(amount) FROM Payments, Orders
                WHERE YEAR(orderDate) = 2003
                AND Payments.customerNumber = Orders.customerNumber
                    GROUP BY MONTH(orderDate));

    SELECT @2004 = (SELECT sum(amount) FROM Payments, Orders
                WHERE YEAR(orderDate) = 2004
                AND Payments.customerNumber = Orders.customerNumber
                    GROUP BY MONTH(orderDate));

    SELECT MONTH(orderDate), (@2004 - @2003) AS Diff 
           FROM Payments, Orders
              WHERE Orders.customerNumber = Payments.customerNumber
                 Group By MONTH(orderDate);

在输出中，我得到了几个月，但对于Diff，我得到了NULL，请帮助。谢谢

星星般的531

我无法测试此数据，因为我没有您的表格，但是请尝试以下操作：

SELECT a.orderMonth, (a.orderTotal - b.orderTotal ) AS Diff 
FROM 
    (SELECT MONTH(orderDate) as orderMonth,sum(amount) as orderTotal 
    FROM Payments, Orders
    WHERE YEAR(orderDate) = 2004
    AND Payments.customerNumber = Orders.customerNumber
        GROUP BY MONTH(orderDate)) as a,
   (SELECT MONTH(orderDate) as orderMonth,sum(amount) as orderTotal FROM Payments, Orders
    WHERE YEAR(orderDate) = 2003
    AND Payments.customerNumber = Orders.customerNumber
        GROUP BY MONTH(orderDate)) as b
WHERE a.orderMonth=b.orderMonth

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