如何使用mysql連接查找每個類別下的產品總數？

Question

我有2個表 - 分類和產品。我想獲得每個類別下的產品總數？我的表看起來像：

表類別

category_id  category_name 
1    first category 
2    second category 
3    third category 

表產品

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 

我想下面的輸出：

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

我目前使用下面的SQL但它不是我產生正確的結果：

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

你需要使用左側加入成爲

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 ;