为什么在我的“插入... ... 关于重复的密钥更新”中有2行受到影响？

我在下表中为 PRIMARY KEY做 INSERT ... ON DUPLICATE KEY UPDATE:

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    |       |
+------------+---------------------------------+------+-----+---------+-------+

然而，即使这些值应该是唯一的，我看到2行受到影响。

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)

为什么会这样？

剪辑

为了进行比较，请参阅下面的查询:

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