在MySQL表中减去值

Question

我在两个不同的表中有价格，想将它们相减（当前价格-当天价格）并以DESC形式订购它们。我想知道是否可以使用单个MySQL命令来完成。

表结构

Table 1
id | Item Name | Date       | Price
 1 | alpha     | 2011-10-05 | 10
 2 | beta      | 2011-10-05 | 12
 3 | gamma     | 2011-10-05 | 14

Table 2
id | Item Name | Date       | Price
 1 | alpha     | 2011-10-04 | 8
 2 | beta      | 2011-10-04 | 10
 3 | gamma     | 2011-10-04 | 12
 4 | alpha     | 2011-10-03 | 4
 5 | beta      | 2011-10-03 | 6
 6 | gamma     | 2011-10-03 | 8

Answer 1

SELECT 
table1.id, table1.`Item Name`,
table1.`Date` AS CurrDate, table1.Price AS CurrPrice,
table2.`Date` AS PrevDate, table2.Price AS PrevPrice,
table1.Price - table2.Price AS Difference
FROM table1
LEFT JOIN table2 ON table1.id = table2.id AND table1.`Date` - INTERVAL 1 DAY = table2.`Date`
ORDER BY Difference DESC

除了我使用LEFT JOIN的方式外，此查询没有什么特别的。我相信，如果无法提供昨天的记录价格，则最后三列将包含NULL。输出：

id | Item Name | CurrDate   | CurrPrice | PrevDate   | PrevPrice | Difference
2  | beta      | 2011-10-05 | 12        | 2011-10-04 | 10        | 2
3  | gamma     | 2011-10-05 | 14        | 2011-10-04 | 12        | 2
1  | alpha     | 2011-10-05 | 10        | 2011-10-04 | 8         | 2