insert時出現主鍵沖突的處理方法

使用"insert into"語句進行數據庫操作時可能遇到主鍵沖突，用戶需要根據應用場景進行忽略或者覆蓋等操作。總結下，有三種解決方案來避免出錯。

測試表：
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

1. insert ignore into
遇主鍵沖突，保持原紀錄，忽略新插入的記錄。

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)

可見 insert ignore into當遇到主鍵沖突時，不更改原紀錄，也不報錯

2. replace into
遇主鍵沖突，替換原紀錄，即先刪除原紀錄，后insert新紀錄

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
其實這個是原本需要執行3條SQL語句（SELECT,INSERT,UPDATE），縮減為1條語句即可完成。
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)

上面語句偽代碼表示即為

if (select * from device where devid=1) { 
    update device set status ='drain' where devid=1 
} else {
    insert into device values (1,'readonly','yang')
}

很明顯，devid=1 是有的，這樣就執行update操作

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

如果更新多條記錄可以向下面這樣寫

insert into device values(1, "dead", "ccc") ON DUPLICATE KEY UPDATE status ='drain', spec_char = "cccc"