原文出处：http://hi.baidu.com/ytjwt/blog/item/1ccc2c26022b0608908f9d8c.html

使用"insert into"语句进行数据库操作时可能遇到主键冲突，用户需要根据应用场景进行忽略或者覆盖等操作。总结下，有三种解决方案来避免出错。

1. insert ignore into
遇主键冲突，保持原纪录，忽略新插入的记录。
mysql> select * from device ;
+-------+--------+-------------+
| devid | status | spec_char   |
+-------+--------+-------------+
|     1 | dead   | zhonghuaren | 
|     2 | dead   | zhong       | 
+-------+--------+-------------+
2 rows in set (0.00 sec)

mysql> insert into device values (1,'alive','yangting');
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'

mysql> insert ignore  into device values (1,'alive','yangting');
Query OK, 0 rows affected (0.00 sec)

mysql> select * from device ;
+-------+--------+-------------+
| devid | status | spec_char   |
+-------+--------+-------------+
|     1 | dead   | zhonghuaren | 
|     2 | dead   | zhong       | 
+-------+--------+-------------+
2 rows in set (0.00 sec)
可见 insert ignore  into当遇到主键冲突时，不更改原纪录，也不报错

2. replace into
遇主键冲突，替换原纪录，即先删除原纪录，后insert新纪录

mysql> replace  into device values (1,'alive','yangting');
Query OK, 2 rows affected (0.00 sec)

mysql> select * from device ;
+-------+--------+-----------+
| devid | status | spec_char |
+-------+--------+-----------+
|     1 | alive  | yangting  | 
|     2 | dead   | zhong     | 
+-------+--------+-----------+
2 rows in set (0.00 sec)

3. insert into ... ON DUPLICATE KEY UPDATE
其实这个是原本需要执行3条SQL语句（SELECT,INSERT,UPDATE），缩减为1条语句即可完成。
即
IF (SELECT * FROM where 存在) {
    UPDATE  SET  WHERE ;
} else {
    INSERT INTO;
}
如：mysql> insert into device values (1,'readonly','yang') ON DUPLICATE KEY UPDATE status ='drain';
Query OK, 2 rows affected (0.00 sec)

上面语句伪代码表示即为
if (select * from device where devid=1) { 
    update device set status ='drain' where devid=1 
} else {
    insert into device values (1,'readonly','yang')
}
很明显，devid=1  是有的，这样就执行update操作
mysql> select * from device ;
+-------+--------+-----------+
| devid | status | spec_char |
+-------+--------+-----------+
|     1 | drain  | yangting  | 
|     2 | dead   | zhong     | 
+-------+--------+-----------+
2 rows in set (0.00 sec)

测试表：
CREATE TABLE `device` (
`devid` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
`status` enum('alive','dead','down','readonly','drain') DEFAULT NULL,
`spec_char` varchar(11) DEFAULT '0',
PRIMARY KEY (`devid`)
) ENGINE=InnoDB