1. 说明

在对带有AUTO_INCREMENT列的表delete掉所有数据时：

（1）对于MyISAM表，在delete表中所有数据时没有任何风险，随意折腾；

（2）对于InnoDB表，在delete表中所有数据时，是可能有风险的，可能会引入一个大坑，具体看后面的实验。

环境描述：RHEL 6.4 x86_64 + MySQL 5.6.19

blog地址：http://blog.csdn.net/hw_libo/article/details/40149173

在维护有AUTO_INCREMENT列的表时，另外一个注意点，参考：

MySQL使用AUTO_INCREMENT列的表注意事项之delete数据篇

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

2. MyISAM表

MySQL [bosco]> CREATE TABLE `t7` (
    ->   `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
    ->   `name` varchar(20) NOT NULL DEFAULT '',
    ->   PRIMARY KEY (`id`)
    -> ) ENGINE=MyISAM;
Query OK, 0 rows affected (0.05 sec)

MySQL [bosco]> insert into t7(name) values('GZ'),('SH'),('BJ'),('SZ'),('HZ');
Query OK, 5 rows affected (0.03 sec)
Records: 5  Duplicates: 0  Warnings: 0

MySQL [bosco]> select * from t7;
+----+------+
| id | name |
+----+------+
|  1 | GZ   |
|  2 | SH   |
|  3 | BJ   |
|  4 | SZ   |
|  5 | HZ   |
+----+------+
5 rows in set (0.00 sec)

MySQL [bosco]> delete from t7;
Query OK, 5 rows affected (0.03 sec)

MySQL [bosco]> show create table t7\G
*************************** 1. row ***************************
       Table: t7
Create Table: CREATE TABLE `t7` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(20) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=6 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

MySQL [bosco]> insert into t7(name) values('NJ');
Query OK, 1 row affected (0.07 sec)

MySQL [bosco]> select * from t7;
+----+------+
| id | name |
+----+------+
|  6 | NJ   |
+----+------+
1 row in set (0.00 sec)


如果在delete from t2后，重启了mysqld，AUTO_INCREMENT并不会被重置：
MySQL [bosco]> show create table t7\G
*************************** 1. row ***************************
       Table: t7
Create Table: CREATE TABLE `t7` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(20) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=6 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

3. InnoDB表

MySQL [bosco]> show create table t2\G
*************************** 1. row ***************************
       Table: t2
Create Table: CREATE TABLE `t2` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(20) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

MySQL [bosco]> insert into t2(name) values('GZ'),('SH'),('BJ'),('SZ'),('HZ');
Query OK, 5 rows affected (0.04 sec)
Records: 5  Duplicates: 0  Warnings: 0


MySQL [bosco1]> select * from t2;
+----+------+
| id | name |
+----+------+
|  1 | GZ   |
|  2 | SH   |
|  3 | BJ   |
|  4 | SZ   |
|  5 | HZ   |
+----+------+
5 rows in set (0.00 sec)


MySQL [bosco]> delete from t2;    ## 直接将t2表中的数据全部删除，使用的是delete而非truncate
Query OK, 5 rows affected (0.04 sec)


MySQL [bosco]> insert into t2(name) values('NJ');   ## 再次插入数据时，自增id会自动为下一编号
Query OK, 1 row affected (0.02 sec)


MySQL [bosco]> select * from t2;
+----+------+
| id | name |
+----+------+
|  6 | NJ   |
+----+------+
1 row in set (0.00 sec)


但是，如果在delete from t2后，重启了mysqld，那么情况完全不一样了，这个重启会重置id。
MySQL [bosco]> insert into t2(name) values('FJ');   ## 再次插入数据时，自增id会被重置编号
Query OK, 1 row affected (0.02 sec)


MySQL [bosco]> select * from t2;
+----+------+
| id | name |
+----+------+
|  1 | FJ   |  ## 再次插入新值时，id编号会重新从头开始
+----+------+
1 row in set (0.00 sec)

blog地址：http://blog.csdn.net/hw_libo/article/details/40149173

-- Bosco QQ：375612082

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