使用子查询选择最小值

Question

我需要编写一个查询，从每个商家那里获取最低价格，并输出价格链接（p_link）和一些其他信息，例如商家名称+评级。

我们有一个价格表 (tgmp_prices)、一个商家表 (tgmp_merchants) 和一个附属产品 ID 表 (tgmp_affiliates)

我已经获得了获取我需要的所有价格和数据的查询：

SELECT p_id, p_m_id, p_prod_label, a_platform, p_link, p_price, m_title, m_link, m_avg_rating 
FROM tgmp_affiliates ga 
JOIN tgmp_prices p 
    ON ga.a_code = p.p_gtin 
        AND ga.a_code != '' 
JOIN tgmp_merchants m 
    ON m.m_id = p.p_m_id 
WHERE ga.site_id = '34' 
    AND p.site_id = '34' 
    AND ga.a_parent = '25573' 
    AND p_type = 'games' 
    AND m.m_hide = 0 
ORDER BY p.p_price ASC

我只需要按商家 ID（p.p_m_id 或 m.m_id）对价格进行分组，然后为每个商家选择最低价格。

SELECT p_id, p_m_id, p_prod_label, a_platform, p_link, p_price, m_title, m_link, m_avg_rating 
FROM tgmp_affiliates ga 
JOIN tgmp_prices p 
    ON ga.a_code = p.p_gtin 
        AND ga.a_code != '' 
JOIN tgmp_merchants m 
    ON m.m_id = p.p_m_id 
WHERE ga.site_id = '34' 
    AND p.site_id = '34' 
    AND ga.a_parent = '25573' 
    AND p_type = 'games' 
    AND m.m_hide = 0 
GROUP BY m.m_id
ORDER BY p.p_price ASC

如果我在 ORDER BY 之前添加 GROUP BY m.m_id，我会得到 16 个结果，每个商家 1 个。我只需要使这个价格最低。所以我尝试添加一个子查询，其中价格表（tgmp_prices）是这样连接的：

SELECT p_id, p_m_id, p_prod_label, a_platform, p_link, p_price, m_title, m_link, m_avg_rating 
FROM tgmp_affiliates ga 
JOIN tgmp_prices p 
    ON ga.a_code = p.p_gtin 
        AND ga.a_code != '' 
        AND p.p_price = ( 
            SELECT MIN(p.p_price) 
            FROM tgmp_prices 
            WHERE p.p_m_id = m.m_id
        ) 
JOIN tgmp_merchants m 
    ON m.m_id = p.p_m_id 
WHERE ga.site_id = '34' 
    AND p.site_id = '34' 
    AND ga.a_parent = '25573' 
    AND p_type = 'games' 
    AND m.m_hide = 0 
GROUP BY m.m_id
ORDER BY p.p_price ASC

这并没有给我正确的结果 - 我认为问题可能是运行子查询时 m.m_id 不可用，所以我尝试在子查询中加入商家表，但我仍然会得到相同的结果。

DDL如下：

CREATE TABLE IF NOT EXISTS `tgmp_affiliates` (
    `a_id` int(8) NOT NULL AUTO_INCREMENT,
    `site_id` int(6) NOT NULL,
    `a_parent` int(8) NOT NULL,
    `a_code` varchar(32) NOT NULL,
    `a_type` varchar(32) NOT NULL,
    `a_platform` varchar(12) NOT NULL,
    PRIMARY KEY (`a_id`),
    UNIQUE KEY `site_id` (`site_id`,`a_parent`,`a_code`,`a_type`,`a_platform`),
    KEY `a_code` (`a_code`),
    KEY `a_parent` (`a_parent`),
    KEY `a_platform` (`a_platform`),
    KEY `a_type` (`a_type`),
    KEY `site_id_2` (`site_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 ;

-- --------------------------------------------------------

--
-- Table structure for table `tgmp_merchants`
--

CREATE TABLE IF NOT EXISTS `tgmp_merchants` (
    `m_id` int(8) NOT NULL AUTO_INCREMENT,
    `site_id` int(6) NOT NULL,
    `m_title` varchar(128) NOT NULL,
    `m_url` text NOT NULL,
    `m_link` varchar(128) NOT NULL,
    `m_favicon` text NOT NULL,
    `m_avg_rating` tinyint(1) NOT NULL,
    `m_hide` tinyint(1) NOT NULL DEFAULT '0',
    PRIMARY KEY (`m_id`),
    UNIQUE KEY `m_title` (`m_title`,`m_link`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 ;

-- --------------------------------------------------------

--
-- Table structure for table `tgmp_prices`
--

CREATE TABLE IF NOT EXISTS `tgmp_prices` (
    `p_id` int(8) NOT NULL AUTO_INCREMENT,
    `site_id` int(6) NOT NULL,
    `p_m_id` int(8) NOT NULL,
    `p_prod_label` varchar(128) NOT NULL,
    `p_gtin` varchar(64) NOT NULL,
    `p_parent` int(8) NOT NULL,
    `p_link` text NOT NULL,
    `p_price` decimal(8,2) NOT NULL,
    `p_delivery` decimal(8,2) NOT NULL,
    `p_currencey` varchar(10) NOT NULL,
    `p_avail` varchar(32) NOT NULL,
    `p_condition` varchar(32) NOT NULL,
    `p_when` datetime NOT NULL,
    `p_type` varchar(32) NOT NULL,
    `p_hide` tinyint(1) NOT NULL DEFAULT '0',
    PRIMARY KEY (`p_id`),
    UNIQUE KEY `p_gtin` (`p_gtin`,`p_m_id`,`p_price`,`p_delivery`),
    KEY `p_price` (`p_price`),
    KEY `p_parent` (`p_parent`),
    KEY `site_id` (`site_id`),
    KEY `p_gtin_2` (`p_gtin`),
    KEY `p_type` (`p_type`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 ;

我现在已将查询修改为以下内容：

SELECT p_id, p_m_id, p_prod_label, a_platform, p_link, p_price, m_title, m_link, m_avg_rating 
FROM tgmp_affiliates ga 
JOIN tgmp_prices p 
    ON ga.a_code = p.p_gtin 
        AND ga.a_code != '' 
        AND p.p_price = (
            SELECT MIN(tp.p_price) FROM tgmp_prices tp
                WHERE tp.p_m_id = m.m_id
                AND tp.p_parent = '25573'
    )
JOIN tgmp_merchants m 
    ON m.m_id = p.p_m_id 
WHERE ga.site_id = '34' 
    AND p.site_id = '34' 
    AND ga.a_parent = '25573' 
    AND p_type = 'games' 
    AND m.m_hide = 0 
GROUP BY m.m_id
ORDER BY p.p_price ASC

这给出了正确数量的结果，它们非常接近正确的结果集，但我仍然没有得到每个商家的最低价格。

Answer 1

也许？：

SELECT p_id,
       p_m_id,
       p_prod_label,
       a_platform,
       p_link,
       p_price,
       m_title,
       m_link,
       m_avg_rating
FROM tgmp_affiliates ga
JOIN tgmp_prices p ON ga.a_code = p.p_gtin
AND ga.a_code != ''
JOIN tgmp_merchants m ON m.m_id = p.p_m_id
WHERE ga.site_id = '34'
  AND p.site_id = '34'
  AND ga.a_parent = '25573'
  AND p_type = 'games'
  AND m.m_hide = 0
  AND p.p_price = (SELECT MIN(p2.p_price) 
                   FROM tgmp_prices p2
                   WHERE p2.p_m_id = m.m_id)
ORDER BY p.p_price ASC

Answer 2

您使用子查询作为标准，而不是将其添加为额外的记录源。这是一个调整：

SELECT p_id, p_m_id, p_prod_label, a_platform
       , p_link, p_price, m_title, m_link, m_avg_rating, MinPrice
FROM tgmp_affiliates ga 
JOIN tgmp_prices p 
    ON ga.a_code = p.p_gtin 
        AND ga.a_code != ''
JOIN tgmp_merchants m 
    ON m.m_id = p.p_m_id 

JOIN
    (
        SELECT tp.p_m_id as lp_m_id, MIN(tp.p_price) AS MinPrice FROM tgmp_prices tp
            WHERE tp.p_parent = '25573'
            GROUP BY tp.p_m_id
    ) LowestPriceForEachMerchant
    ON m.m_id = LowestPriceForEachMerchant.lp_m_id

WHERE ga.site_id = '34' 
    AND p.site_id = '34' 
    AND ga.a_parent = '25573' 
    AND p_type = 'games' 
    AND m.m_hide = 0 
GROUP BY m.m_id
ORDER BY p.p_price ASC