如何使用mysql连接查找每个类别下的产品总数？

Question

我有2张桌子 - 粪便和产品。我想获得每个类别下的产品总数？我的表格如下：

表类别

category_id     category_name
1               first category
2               second category
3               third category

表产品

product_id     product_name     category_id
1              first product    1
2              second product   1
3              third product    1
4              fourth product   3
5              fifth product    3

我想要关注输出：

category_id     category_name     total_products
1               first category    3
2               second category   0
3               third category    2

我目前正在使用以下sql，但它没有让我得到正确的结果：

SELECT `c`.`category_id`, `c`.`category_name`, COUNT(`p`.`product_id`) AS total_products FROM `categories` AS `c` INNER JOIN `products` AS `p` ON `c`.`category_id` = `p`.`product_id` GROUP BY `p`.`category_id`

Answer 1

您需要使用左连接作为

select
c.*, coalesce(count(p.category_id),0) as total_products
from categories c
left join products p on p.category_id = c.category_id
group by c.category_id ;