MySQL-针对一定范围的列选择不同的值 [英] MySQL - select distinct values against a range of columns

问题描述

下表将存储一段时间内各种货币之间的汇率:

The following table will store exchange rates between various currencies over time:

CREATE TABLE `currency_exchange` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `currency_from` int(11) DEFAULT NULL,
 `currency_to` int(11) DEFAULT NULL,
 `rate_in` decimal(12,4) DEFAULT NULL,
 `rate_out` decimal(12,4) DEFAULT NULL,
 `exchange_date` datetime DEFAULT NULL,
 PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

我将如何查询它以获取最新汇率列表?

How would I query it to fetch a list the most recent exchange rates?

从本质上讲，这将标识合并的 currency_from 和 currency_to 列为不同的汇率，然后找到具有最新日期的列.

It'd essentially be identifying the combined currency_from and currency_to columns as a distinct exchange rate, then finding the one with the most recent date.

例如，假设我的数据为:

For example, let's say I've got the data as:

INSERT INTO currency_exchange (id, currency_from, currency_to, rate_in, rate_out, exchange_date) VALUES
(1, 1, 2, 0.234, 1.789, '2012-07-23 09:24:34'),
(2, 2, 1, 0.234, 1.789, '2012-07-24 09:24:34'),
(3, 2, 1, 0.234, 1.789, '2012-07-24 09:24:35'),
(4, 1, 3, 0.234, 1.789, '2012-07-24 09:24:34');

我希望它选择行ID:

• 1-作为货币1和2之间的最新汇率
• 3-作为货币2和1之间的最新汇率
• 4-作为货币1和3之间的最新汇率

推荐答案

以下查询应该有效:

SELECT ce.*
FROM currency_exhcnage ce
LEFT JOIN currency_exchange newer
    ON (newer.currency_from = ce.currency_from
    AND newer.currency_to = ce.currency_to
    AND newer.exchange_date > ce.exchange_date)
WHERE newer.id IS NULL

进行自我 LEFT JOIN 的技巧是避免求助于子查询，如果您有大量数据集，子查询可能会非常昂贵.本质上是在寻找不存在较新"记录的记录.

The trick of doing a self-LEFT JOIN is to avoid resorting to a subquery that may be very expensive if you have large datasets. It's essentially looking for records where no "newer" record exists.

或者，您可以选择一个更简单的方法(尽管它可能(或可能不会，如注释中所述)比较慢):

Alternatively, you could go for a simpler (although it may (or may not, as noted in comments) be slower):

SELECT *
FROM currency_exchange ce
NATURAL JOIN (
    SELECT currency_from, currency_to, MAX(exchange_date) AS exchange_date
    FROM currency_exchange
    GROUP BY currency_from, currency_to
) AS most_recent

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