请问老师AUTO_INCREMENT=1;为什么不行啊

 user1 | CREATE TABLE `user1` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT '编号',
  `username` varchar(20) NOT NULL COMMENT '用户',
  `age` tinyint(3) unsigned NOT NULL DEFAULT '18' COMMENT '年龄',
  `sex` enum('男','女','保密') NOT NULL DEFAULT '保密' COMMENT '性别',
  `addr` varchar(20) NOT NULL DEFAULT '北京',
  `married` tinyint(1) NOT NULL DEFAULT '0' COMMENT '0代表未结婚，1代表已 结婚',
  `salary` float(8,2) NOT NULL DEFAULT '0.00' COMMENT '薪水',
  `userDesc` varchar(100) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `username` (`username`)
) ENGINE=InnoDB AUTO_INCREMENT=15 DEFAULT CHARSET=utf8 |
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> ALTER TABLE user1 AUTO_INCREMENT=1;
Query OK, 0 rows affected (0.10 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SHOW CREATE TABLE user1;
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| user1 | CREATE TABLE `user1` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT '编号',
  `username` varchar(20) NOT NULL COMMENT '用户',
  `age` tinyint(3) unsigned NOT NULL DEFAULT '18' COMMENT '年龄',
  `sex` enum('男','女','保密') NOT NULL DEFAULT '保密' COMMENT '性别',
  `addr` varchar(20) NOT NULL DEFAULT '北京',
  `married` tinyint(1) NOT NULL DEFAULT '0' COMMENT '0代表未结婚，1代表已 结婚',
  `salary` float(8,2) NOT NULL DEFAULT '0.00' COMMENT '薪水',
  `userDesc` varchar(100) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `username` (`username`)
) ENGINE=InnoDB AUTO_INCREMENT=12 DEFAULT CHARSET=utf8 |
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> ALTER TABLE user1 AUTO_INCREMENT=3;
Query OK, 0 rows affected (0.07 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SHOW CREATE TABLE user1;
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| user1 | CREATE TABLE `user1` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT '编号',
  `username` varchar(20) NOT NULL COMMENT '用户',
  `age` tinyint(3) unsigned NOT NULL DEFAULT '18' COMMENT '年龄',
  `sex` enum('男','女','保密') NOT NULL DEFAULT '保密' COMMENT '性别',
  `addr` varchar(20) NOT NULL DEFAULT '北京',
  `married` tinyint(1) NOT NULL DEFAULT '0' COMMENT '0代表未结婚，1代表已 结婚',
  `salary` float(8,2) NOT NULL DEFAULT '0.00' COMMENT '薪水',
  `userDesc` varchar(100) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `username` (`username`)
) ENGINE=InnoDB AUTO_INCREMENT=12 DEFAULT CHARSET=utf8 |
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.02 sec)