Doubts about queries in MySQL - mysql

Question

I wanted to make a query with Rails, something like this:
filters = Filter.joins(:category_filters).where("category_id IN (?)", params[:categories]).group("filters.id")
And the MySQL statement that is making is this:
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
At first sight, this query is ok, but when I look the results, its wrong. Let me explain you.
First, this is a query to the table filters:
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 |
+----+----------+-------+----------+------------+------------+
Second, this is a query to the table category_filters:
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 |
+----+-----------+-------------+------------+------------+
And now, the query generated by Rails (the first query):
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 |
+----+----------+-------+----------+------------+------------+
Why is this happening?
But now, this is similar query, instead of using IN I used OR, like this:
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 |
+----+----------+-------+----------+------------+------------+
What is happening?

Answer 1

In your WHERE clause, try this (assuming category_id is a number):
category_id IN (9,4)
else this (assuming category_id is a string)
category_id IN ('9','4')
instead of this (in your original query)
category_id IN ('9,4')

Answer 2

If you just do
where(:category_id => params[:categories]
Rails will create the proper SQL syntax for you