mySQL从同列中划分两个值

Question

我希望使用mySQL来划分同一列的值。

有没有更好的方法做到这一点???我真的需要3个选择语句吗？

SELECT (SELECT FixAM From Fixes WHERE Id = 1)/(SELECT FixAM From Fixes WHERE Id = 2) 

我的表结构如下：

CREATE TABLE IF NOT EXISTS `Fixes` (
    `Id` int(11) NOT NULL AUTO_INCREMENT COMMENT 'PK', 
    `CurrencyId` int(11) NOT NULL COMMENT 'FK', 
    `MetalId` int(11) NOT NULL COMMENT 'FK', 
    `FixAM` decimal(10,5) NOT NULL, 
    `FixPM` decimal(10,5) DEFAULT NULL, 
    `TimeStamp` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP, 
    PRIMARY KEY (`Id`), 
    KEY `CurrencyId` (`CurrencyId`), 
    KEY `MetalId` (`MetalId`) 
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci AUTO_INCREMENT=13 ; 

-- 
-- Dumping data for table `Fixes` 
-- 

INSERT INTO `Fixes` (`Id`, `CurrencyId`, `MetalId`, `FixAM`, `FixPM`, `TimeStamp`) VALUES 
(1, 1, 1, '1592.50000', '1586.25000', '2013-02-25 15:10:21'), 
(2, 2, 1, '1051.84900', '1049.59300', '2013-02-25 15:10:21'), 
(3, 3, 1, '1201.88700', '1194.10600', '2013-02-25 15:10:21'), 
(4, 1, 2, '29.17000', NULL, '2013-02-25 13:54:02'), 
(5, 2, 2, '19.27580', NULL, '2013-02-25 13:54:02'), 
(6, 3, 2, '21.98190', NULL, '2013-02-25 13:54:02'), 
(7, 1, 3, '1627.00000', '1620.00000', '2013-02-25 14:28:59'), 
(8, 2, 3, '1074.65000', '1072.50000', '2013-02-25 14:28:59'), 
(9, 3, 3, '1229.30000', '1218.95000', '2013-02-25 14:28:59'), 
(10, 1, 4, '747.00000', '748.00000', '2013-02-25 14:28:59'), 
(11, 2, 4, '493.40000', '495.20000', '2013-02-25 14:28:59'), 
(12, 3, 4, '564.40000', '562.85000', '2013-02-25 14:28:59'); 

Answer 1

我认为这是你在找什么：

SELECT MetalId, 
    MAX(CASE WHEN CurrencyId = 1 THEN FixAM END)/
    MAX(CASE WHEN CurrencyId = 2 THEN FixAM ELSE 1 END) Output 
FROM Fixes 
GROUP BY MetalId 

这将产生1592.50000/1051.849000。如果你想要相反的话，交换货币ids。

SQL Fiddle Demo

如果你没有CurrencyId = 2，I分割值默认为1这样你就不会收到错误。