1. 说明

(1)对于MyISAM表,如果用UPDATE更新自增列,如果列值与已有的值重复,则会出错;如果大于已有的最大值,则会自动更新表的AUTO_INCREMENT,操作是安全的。

(2)对于innodb表,update auto_increment字段,如果列值与已有的值重复,则会出错;如果大于已有的最大值,可能会引入一个坑,会造成编号重复错误,插入数据失败的情况,可见在update自增列值是要注意。

环境描述:RHEL 6.4 x86_64 + MySQL 5.6.19

blog地址:http://blog.csdn.net/hw_libo/article/details/40097125

下面实验证实:

2. MyISAM表

MySQL [bosco]> CREATE TABLE `t5` (
    ->   `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
    ->   PRIMARY KEY (`id`)
    -> ) ENGINE=MyISAM;
Query OK, 0 rows affected (0.05 sec)


MySQL [bosco]> insert into t5 values(null);
Query OK, 1 row affected (0.07 sec)


MySQL [bosco]> select * from t5;
+----+
| id |
+----+
|  1 |
+----+
1 row in set (0.00 sec)


MySQL [bosco]> insert into t5 values(5),(9);
Query OK, 2 rows affected (0.04 sec)
Records: 2  Duplicates: 0  Warnings: 0


MySQL [bosco]> select * from t5;
+----+
| id |
+----+
|  1 |
|  5 |
|  9 |
+----+
3 rows in set (0.00 sec)

2.1 MyISAM表update自增列,由大改小


MySQL [bosco]> show create table t5\G
*************************** 1. row ***************************
       Table: t5
Create Table: CREATE TABLE `t5` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=10 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)


MySQL [bosco]> update t5 set id=4 where id=9;   ## 将自增列由大改小,没有问题
Query OK, 1 row affected (0.02 sec)
Rows matched: 1  Changed: 1  Warnings: 0


MySQL [bosco]> show create table t5\G
*************************** 1. row ***************************
       Table: t5
Create Table: CREATE TABLE `t5` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=10 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)


2.2 MyISAM表update自增列,由小改大


MySQL [bosco]> show create table t5\G
*************************** 1. row ***************************
       Table: t5
Create Table: CREATE TABLE `t5` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=10 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)


MySQL [bosco]> update t5 set id=12 where id=5;   ## 将自增列由小改大,而且大于当前的AUTO_INCREMENT,同样是没有问题
Query OK, 1 row affected (0.03 sec)
Rows matched: 1  Changed: 1  Warnings: 0


MySQL [bosco]> show create table t5\G
*************************** 1. row ***************************
       Table: t5
Create Table: CREATE TABLE `t5` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=13 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
这里自动修改最新的auto_increment变为13。

可见,MyISAM表的update自增列不会存在风险。

3. InnoDB表


MySQL [bosco]> CREATE TABLE `t6` (
    ->   `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
    ->   PRIMARY KEY (`id`)
    -> ) ENGINE=InnoDB;
Query OK, 0 rows affected (0.06 sec)


MySQL [bosco]> insert into t6 values(null);
Query OK, 1 row affected (0.05 sec)


MySQL [bosco]> insert into t6 values(5),(9);
Query OK, 2 rows affected (0.01 sec)
Records: 2  Duplicates: 0  Warnings: 0


MySQL [bosco]> select * from t6;
+----+
| id |
+----+
|  1 |
|  5 |
|  9 |
+----+
3 rows in set (0.00 sec)


3.1 InnoDB表update自增列,由大改小


MySQL [bosco]> show create table t6\G
*************************** 1. row ***************************
       Table: t6
Create Table: CREATE TABLE `t6` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)


MySQL [bosco]> update t6 set id=4 where id=9;
Query OK, 1 row affected (0.04 sec)
Rows matched: 1  Changed: 1  Warnings: 0


MySQL [bosco]> show create table t6\G
*************************** 1. row ***************************
       Table: t6
Create Table: CREATE TABLE `t6` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
可见,InnoDB表update自增列时,由大值改为小值,除了可能会出现重复数据修改失败外,没有其他风险。

3.2 InnoDB表update自增列,由小改大

MySQL [bosco]> select * from t6;
+----+
| id |
+----+
|  1 |
|  4 |
|  5 |
+----+
3 rows in set (0.00 sec)


MySQL [bosco]> show create table t6\G
*************************** 1. row ***************************
       Table: t6
Create Table: CREATE TABLE `t6` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)


MySQL [bosco]> update t6 set id=12 where id=5;   ## 将自增列由小改大,而且大于当前的AUTO_INCREMENT,这就相当于挖了坑了
Query OK, 1 row affected (0.03 sec)
Rows matched: 1  Changed: 1  Warnings: 0


MySQL [bosco]> select * from t6;
+----+
| id |
+----+
|  1 |
|  4 |
| 12 |
+----+
3 rows in set (0.01 sec)


MySQL [bosco]> show create table t6\G
*************************** 1. row ***************************
       Table: t6
Create Table: CREATE TABLE `t6` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8
1 row in set (0.01 sec)
表中自增列最大值已经是12,这个update操作不会自动修改最新的auto_increment变为13,那么这就会有问题,以后增加到12后,就会出现冲突,导致数据插入失败:
MySQL [bosco]> insert into t6 values(null),(null);
Query OK, 2 rows affected (0.03 sec)
Records: 2  Duplicates: 0  Warnings: 0


MySQL [bosco]> insert into t6 values(null);   ## 错误出现了。
ERROR 1062 (23000): Duplicate entry '12' for key 'PRIMARY'

blog地址:http://blog.csdn.net/hw_libo/article/details/40097125

-- Bosco QQ:375612082

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