为什么在我的`INSERT ... ON DUPLICATE KEY UPDATE`中影响了2行？

Question

我正在INSERT ... ON DUPLICATE KEY UPDATE为PRIMARY KEY下表中的a 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,即使这些值应该是唯一的,我也会看到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？

编辑

为了比较,请参阅此查询:

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

Answer 1

从手册:

使用ON DUPLICATE KEY UPDATE时,如果将行作为新行插入,则每行的受影响行值为1;如果更新现有行,则每行受影响的行值为2.

Answer 2

所以你知道你是更新了一行（重复键）还是只是插入了一个：http : //dev.mysql.com/doc/refman/5.0/en/insert-on-duplicate.html