MySQL计算联合表中的行数

Question

我想计算行数，或者在这种情况下计算同一类别中的产品数量，如果该类别中没有产品，则返回零。

我的查询中的表如下所示：

category                    category_lang                 media
------------                -------------                 ---------
category_id | published     category_id | name | alias    media_id | category_id
-----------------------     --------------------------    ----------------------
1            | 1            1           | One  | one      1        | 1
2            | 1            2           | Two  | two      2        | 2
3            | 1            3           | Three| three    3        | 3

media_lang                          product_category
-------------------------------     ----------------
media_id | url      | file_name     product_id | category_id
-------------------------------     ------------------------
1        | /images/ | file1.jpg     1          | 1
2        | /images/ | file2.jpg     2          | 1
3        | /images/ | file3.jpg     3          | 2

我想要这样的结果：

category_id | category_name | alias | media_id | url      | file_name | count
1           | One           | one   | 1        | /images/ | file1.jpg | 2
2           | Two           | two   | 2        | /images/ | file2.jpg | 1
3           | Three         | three | 3        | /images/ | file3.jpg | 0

我的查询目前看起来像这样

SELECT 
c.`category_id`, 
ca.`name`, 
ca.`alias`, 
m.`media_id`, 
ma.`url`, 
ma.`file_name`,
COUNT(p.`product_id`) AS `count`
FROM `category` c
LEFT JOIN `category_lang` ca ON (c.`category_id` = ca.`category_id`)
LEFT JOIN `media` m ON (c.`category_id` = m.`category_id`)
LEFT JOIN `media_lang` ma ON (m.`media_id` = ma.`media_id`)
LEFT JOIN `product_category` p ON (c.`category_id` = p.`category_id`)
WHERE c.`published` = 1
ORDER BY ca.`name`

我的逻辑显然是错误的，因为这个查询会返回这个：

category_id | category_name | alias | media_id | url      | file_name | count
1           | One           | one   | 1        | /images/ | file1.jpg | 3

我怎样才能达到预期的效果？

Answer 1

我认为解决这个问题的最轻松（和理智）的方法是在单独的子查询中汇总 product_category 表中每个类别的产品计数，然后将其加入您已有的：

SELECT 
    c.category_id, 
    ca.name, 
    ca.alias, 
    m.media_id, 
    ma.url, 
    ma.file_name,
    COALESCE(t.cat_count, 0) AS cat_count
FROM category c
LEFT JOIN category_lang ca
    ON c.category_id = ca.category_id
LEFT JOIN media m
    ON c.category_id = m.category_id
LEFT JOIN media_lang ma
    ON m.media_id = ma.media_id
LEFT JOIN
(
    SELECT category_id, COUNT(*) AS cat_count
    FROM product_category
    GROUP BY category_id
) t
    ON c.category_id = t.category_id
WHERE c.published = 1
ORDER BY ca.name

请注意，您的product_category 表没有任何没有产品的类别条目。这不是问题，因为在LEFT JOIN 中，我们可以简单地将NULL 计数视为零。如果给定类别与子查询中的任何内容都不匹配，则会出现 NULL 值。

Answer 2

在我看来，这是您进行计数的地方，并且您确实有进行计数的标准。在类别表中添加产品类别 id 等于类别 id 的额外条件应该可以修复不正确的计数。

SELECT 
c.`category_id`, 
ca.`name`, 
ca.`alias`, 
m.`media_id`, 
ma.`url`, 
ma.`file_name`,
COUNT(p.`product_id`) AS `count`
FROM `category` c
LEFT JOIN `category_lang` ca ON (c.`category_id` = ca.`category_id`)
LEFT JOIN `media` m ON (c.`category_id` = m.`category_id`)
LEFT JOIN `media_lang` ma ON (m.`media_id` = ma.`media_id`)
LEFT JOIN `product_category` p ON (c.`category_id` = p.`category_id`)
WHERE (c.`published` = 1) and (c.`category_id` = p.`category_id`)
ORDER BY ca.`name`