我正在INSERT ... ON DUPLICATE KEY UPDATE为PRIMARY KEY下表做一个：

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

但是，尽管这些值应该是唯一的，但我发现有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)

为什么会这样呢？

编辑

为了进行比较，请参见以下查询：

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