计数子句 -> 不正确的计数值

Question

我们在使用带有内/左连接的计数组合时遇到了一个问题，我们无法弄清楚如何解决。 对于此事，我们将不胜感激！

示例中有 4 个表：

1: providers: Including 2 providers
2: providers_categories: Including 2 categories. 1 provider can be in multiple categories (this seems to be causing the issue)
3: connections_providers: connecting the providers to the categories
4: reviews_providers: currently we have included 1 rating per provider

目标：从表reviews_providers 中输出评论计数。

问题：提供商 2 包含在 2 个类别中。评论计数加倍：每个提供商类别有 1 条计数：即使只有 1 条条目，也会打印总共 2 条评论。

谢谢！

代码：

SELECT prov.id, prov.title, prov_cat.title AS category, AVG(reviews.rating) AS rating, COUNT(reviews.rating) AS count 
FROM connections_providers_categories conn 
INNER JOIN providers_categories prov_cat 
 ON prov_cat.id = conn.category_id 
LEFT JOIN reviews_providers reviews 
 ON reviews.provider_id = conn.provider_id 
INNER JOIN providers prov 
 ON prov.id = conn.provider_id 
GROUP BY prov.id 
ORDER BY prov.title ASC

CREATE TABLE `connections_providers_categories` (
  `provider_id` int(4) UNSIGNED NOT NULL,
  `category_id` int(4) UNSIGNED NOT NULL
) ENGINE=MyISAM DEFAULT;

INSERT INTO `connections_providers_categories` (`provider_id`, `category_id`) VALUES
(1, 1),
(2, 1),
(2, 2);

CREATE TABLE `providers` (
  `id` int(4) UNSIGNED NOT NULL AUTO_INCREMENT,
  `title` varchar(100) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=3 DEFAULT;

INSERT INTO `providers` (`id`, `title`) VALUES
(1, 'Provider 1'),
(2, 'Provider 2');

CREATE TABLE `providers_categories` (
  `id` int(4) UNSIGNED NOT NULL AUTO_INCREMENT,
  `title` varchar(60) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=3 DEFAULT;

INSERT INTO `providers_categories` (`id`, `title`) VALUES
(1, 'Category 1'),
(2, 'Category 2');

CREATE TABLE `reviews_providers` (
  `id` int(4) UNSIGNED NOT NULL AUTO_INCREMENT,
  `provider_id` int(4) UNSIGNED NOT NULL,
  `rating` enum('1','2','3','4','5') DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=3 DEFAULT;

INSERT INTO `reviews_providers` (`id`, `provider_id`, `rating`) VALUES
(1, 2, '5'),
(2, 1, '3');

我们的问题可能类似于以下问题，但我们找不到答案/即使这两个问题都包含多个计数，我们也可以看到它是相同的情况：count is multiplied after adding left join 看来我们可能需要一个子查询，但我们不知道该怎么做。

有什么建议吗？ 谢谢！

Answer 1

你可以使用子查询顶部得到你的结果

SELECT prov.id, prov.title, GROUP_CONCAT(prov_cat.title) AS category, reviews.rating , reviews.count 
FROM connections_providers_categories conn 
INNER JOIN providers_categories prov_cat 
 ON prov_cat.id = conn.category_id 
LEFT JOIN (SELECT  provider_id, AVG(rating) AS rating, COUNT(provider_id) AS count FROM reviews_providers GROUP BY provider_id)  reviews 
 ON reviews.provider_id = conn.provider_id 
INNER JOIN providers prov 
 ON prov.id = conn.provider_id 
GROUP BY prov.id,prov.title
ORDER BY prov.title ASC

编号 |标题 |类别 |评级 |数数 -: | :--------- | :-------------------- | -----: | ----: 1 |提供者 1 |第一类 | 3 | 1 2 |提供者 2 |第 2 类，第 1 类 | 5 | 1

db小提琴here