在表中有数据时重置AUTO_INCREMENT似乎没有效果

控制台输出如下：

mysql> SELECT * FROM user;
+-----+----------+
| id  | username |
+-----+----------+
| 119 | Andy     |
| 118 | Linus    |
| 117 | Mike     |
+-----+----------+
3 rows in set (0.00 sec)

mysql> ALTER TABLE user
    -> AUTO_INCREMENT=1;
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SHOW CREATE TABLE user;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| user  | CREATE TABLE `user` (
  `id` int unsigned NOT NULL AUTO_INCREMENT COMMENT '用户编号',
  `username` varchar(20) NOT NULL COMMENT '用户名',
  PRIMARY KEY (`id`),
  UNIQUE KEY `username` (`username`)
) ENGINE=InnoDB AUTO_INCREMENT=120 DEFAULT CHARSET=utf8 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)

mysql> ALTER TABLE user AUTO_INCREMENT=1;
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SHOW CREATE TABLE user;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| user  | CREATE TABLE `user` (
  `id` int unsigned NOT NULL AUTO_INCREMENT COMMENT '用户编号',
  `username` varchar(20) NOT NULL COMMENT '用户名',
  PRIMARY KEY (`id`),
  UNIQUE KEY `username` (`username`)
) ENGINE=InnoDB AUTO_INCREMENT=120 DEFAULT CHARSET=utf8 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> INSERT user(username) VALUES("John");
Query OK, 1 row affected (0.03 sec)

mysql> SELECT * FROM user;
+-----+----------+
| id  | username |
+-----+----------+
| 119 | Andy     |
| 120 | John     |
| 118 | Linus    |
| 117 | Mike     |
+-----+----------+
4 rows in set (0.00 sec)

mysql> DELETE FROM user;
Query OK, 4 rows affected (0.01 sec)

mysql> ALTER TABLE user AUTO_INCREMENT=1;
Query OK, 0 rows affected (0.07 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SHOW CREATE TABLE user;
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                              |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| user  | CREATE TABLE `user` (
  `id` int unsigned NOT NULL AUTO_INCREMENT COMMENT '用户编号',
  `username` varchar(20) NOT NULL COMMENT '用户名',
  PRIMARY KEY (`id`),
  UNIQUE KEY `username` (`username`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> INSERT USER(username) VALUES("Mike"),("Linus"),("Andy");
Query OK, 3 rows affected (0.06 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> SHOW CREATE TABLE user;
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                               |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| user  | CREATE TABLE `user` (
  `id` int unsigned NOT NULL AUTO_INCREMENT COMMENT '用户编号',
  `username` varchar(20) NOT NULL COMMENT '用户名',
  PRIMARY KEY (`id`),
  UNIQUE KEY `username` (`username`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> DELETE FROM user;
Query OK, 3 rows affected (0.01 sec)

mysql> SHOW CREATE TABLE user;
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                               |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| user  | CREATE TABLE `user` (
  `id` int unsigned NOT NULL AUTO_INCREMENT COMMENT '用户编号',
  `username` varchar(20) NOT NULL COMMENT '用户名',
  PRIMARY KEY (`id`),
  UNIQUE KEY `username` (`username`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql>