mysql - MySQL中查询的疑惑

Question

我想用 Rails 做一个查询，像这样:

filters = Filter.joins(:category_filters).where("category_id IN (?)", params[:categories]).group("filters.id")

MySQL 的语句是这样的:

SELECT `filters`.* FROM `filters` INNER JOIN `category_filters` ON `category_filters`.`filter_id` = `filters`.`id` WHERE (category_id IN ('9,4')) GROUP BY filters.id

乍一看，这个查询没问题，但当我看结果时，它是错误的。让我来解释一下。

首先，这是对表过滤器的查询:

select * from filters;

+----+----------+-------+----------+------------+------------+
| id | name     | other | optional | created_at | updated_at |
+----+----------+-------+----------+------------+------------+
|  1 | material |     1 |        1 | NULL       | NULL       |
|  2 | abc      |     1 |        0 | NULL       | NULL       |
|  3 | xyz      |     0 |        0 | NULL       | NULL       |
|  4 | 123a     |     0 |        0 | NULL       | NULL       |
+----+----------+-------+----------+------------+------------+

其次，这是对表 category_filters 的查询:

select * from category_filters;

+----+-----------+-------------+------------+------------+
| id | filter_id | category_id | created_at | updated_at |
+----+-----------+-------------+------------+------------+
|  1 |         1 |           1 | NULL       | NULL       |
|  2 |         2 |           1 | NULL       | NULL       |
|  3 |         1 |           9 | NULL       | NULL       |
|  4 |         2 |           9 | NULL       | NULL       |
|  5 |         1 |           4 | NULL       | NULL       |
|  6 |         3 |           4 | NULL       | NULL       |
+----+-----------+-------------+------------+------------+

现在，由 Rails 生成的查询(第一个查询):

SELECT `filters`.* FROM `filters` INNER JOIN `category_filters` ON `category_filters`.`filter_id` = `filters`.`id` WHERE (category_id IN ('9,4')) GROUP BY filters.id;

+----+----------+-------+----------+------------+------------+
| id | name     | other | optional | created_at | updated_at |
+----+----------+-------+----------+------------+------------+
|  1 | material |     1 |        1 | NULL       | NULL       |
|  2 | abc      |     1 |        0 | NULL       | NULL       |
+----+----------+-------+----------+------------+------------+

为什么会这样？

但是现在，这是类似的查询，我没有使用 IN，而是使用了 OR，如下所示:

SELECT `filters`.* FROM `filters` INNER JOIN `category_filters` ON `category_filters`.`filter_id` = `filters`.`id` WHERE (category_filters.category_id=9 or category_filters.category_id=4) GROUP BY filters.id;

+----+----------+-------+----------+------------+------------+
| id | name     | other | optional | created_at | updated_at |
+----+----------+-------+----------+------------+------------+
|  1 | material |     1 |        1 | NULL       | NULL       |
|  2 | abc      |     1 |        0 | NULL       | NULL       |
|  3 | xyz      |     0 |        0 | NULL       | NULL       |
+----+----------+-------+----------+------------+------------+

发生了什么事？

Answer 1

在您的 WHERE 子句中，试试这个(假设 category_id 是一个数字):

category_id IN (9,4)

否则这个(假设category_id是一个字符串)

category_id IN ('9','4')

而不是这个(在您的原始查询中)

category_id IN ('9,4')