根据条件将两个表以及第二个表中的记录数联接在一起

问题描述

我有以下表格-

这里是 SQLFIDDLE

类别

+-----------+-------------+
|  column   |    type     |
+-----------+-------------+
| id        | int(11)     |
| name      | varchar(40) |
| unit      | varchar(50) |
| is_active | tinyint(1)  |
+-----------+-------------+

和

产品

+-------------+---------------+
|   column    |     type      |
+-------------+---------------+
| id          | int(11)       |
| category_id | int(11)       |
| name        | varchar(40)   |
| base_rate   | decimal(10,2) |
| is_active   | tinyint(1)    |
+-------------+---------------+

我想获取类别列表以及有效产品的数量.如果某个类别的产品均未激活，则应返回0.

I want to get list of categories along with count of number of products active. If no products are active for a category it should return 0.

有点像下表-

+----+--------+--------------+
| id |  name  | active_count |
+----+--------+--------------+
|  1 | Steel  |            1 |
|  2 | Cement |            2 |
+----+--------+--------------+

我提出了以下查询-

SELECT c.id, c.name, c.unit, COUNT(p.category_id) as active_count 
FROM `categories` c 
JOIN `products` p
    ON c.id = p.category_id
WHERE ( p.is_active = 1 )
GROUP BY p.category_id;

仅当每个类别中至少有一个活跃的产品时，此以上查询才有效.如果没有可用的产品，则应返回active_count为 0

This above query works only when there is at least one product active in each of category. If there are not products available it should return active_count as 0

我该如何解决?

这里是 SQLFIDDLE

推荐答案

使用LEFT JOIN代替INNER JOIN:

SELECT c.id, c.name, c.unit, COUNT(p.category_id) as active_count 
FROM `categories` c 
LEFT JOIN `products` p
    ON c.id = p.category_id AND p.is_active = 1 
GROUP BY c.id;

将谓词p.is_active = 1从WHERE子句移到ON也很重要，以便查询返回categories表的 all 条记录.

It is also important to move predicate p.is_active = 1 from WHERE clause to ON, so that all records of categories table are returned by the query.

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