mysql delete auto_increment列时的注意问题

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。（delete 完不能重启mysql）

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)