Why are 2 rows affected in my `INSERT ... ON DUPLICATE KEY UPDATE`?

Question

I'm doing an INSERT ... ON DUPLICATE KEY UPDATE for a PRIMARY KEY in the following table:

DESCRIBE users_interests; 

+------------+---------------------------------+------+-----+---------+-------+ | Field      | Type                            | Null | Key | Default | Extra | +------------+---------------------------------+------+-----+---------+-------+ | uid        | int(11)                         | NO   | PRI | NULL    |       | | iid        | int(11)                         | NO   | PRI | NULL    |       | | preference | enum('like','dislike','ignore') | YES  |     | NULL    |       | +------------+---------------------------------+------+-----+---------+-------+ 

However, even though these values should be unique, I'm seeing 2 rows affected.

INSERT INTO users_interests (uid, iid, preference) VALUES (2, 2, 'like') ON DUPLICATE KEY UPDATE preference='like'; 

Query OK, 2 rows affected (0.04 sec) 

Why is this happening?

EDIT

For comparison, see this query:

UPDATE users_interests SET preference='like' WHERE uid=2 AND iid=2; 

Query OK, 1 row affected (0.44 sec) Rows matched: 1  Changed: 1  Warnings: 0 

Answer 1

From the manual:

With ON DUPLICATE KEY UPDATE, the affected-rows value per row is 1 if the row is inserted as a new row and 2 if an existing row is updated.

Answer 2

So you know whether you updated a row (duplicate key) or just inserted one: http://dev.mysql.com/doc/refman/5.0/en/insert-on-duplicate.html