Question

I am working with a many-to-many relationship in MySQL as in the simplified example below. What I would like to do is given a category id, find the objects that are in that category and ONLY that category.

I can easily get all of the objects in a category:

SELECT * from object INNER JOIN link ON object.objectID = link.objectID WHERE link.categoryID=1;

This gives me object1 and object2, however what I want is just object1 as object2 is also in category2.

The only thing I could think of was using this as a subquery to get all the links for those object ids and then getting the ones with a count of 1.

I’m hoping there is a simpler and more efficient way!

-- Table: object
+---------+-----------+
| id      | name      |
+---------+-----------+
|       1 | object1   |
|       2 | object2   |
+---------+-----------+

-- Table: category
+------+-----------+
| id   | name      |
+------+-----------+
|    1 | category1 |
|    2 | category2 |
+------+---------+

-- Table: link
+-----------+-------------+
| objectid  | categoryid  |
+-----------+-------------+
|    1      | 1           |
|    2      | 1           |
|    2      | 2           |
+-----------+-------------+

Answer 1

You can group the link table by object and filter for only those groups containing one record:

SELECT objectid FROM link GROUP BY objectid HAVING COUNT(*) = 1 AND categoryid = 1