更改 int 字段为 unsigned 后默认宽度就从 11 变成 10 是什么原因

如下

mysql> desc a1;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.23 sec)

mysql> alter table a1 modify id int zerofill;
Query OK, 0 rows affected (0.55 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc a1;
+-------+---------------------------+------+-----+---------+-------+
| Field | Type                      | Null | Key | Default | Extra |
+-------+---------------------------+------+-----+---------+-------+
| id    | int(10) unsigned zerofill | YES  |     | NULL    |       |
| name  | varchar(20)               | YES  |     | NULL    |       |
+-------+---------------------------+------+-----+---------+-------+
2 rows in set (0.03 sec)

mysql> alter table a1 modify id int(11) zerofill;
Query OK, 0 rows affected (0.36 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc a1;
+-------+---------------------------+------+-----+---------+-------+
| Field | Type                      | Null | Key | Default | Extra |
+-------+---------------------------+------+-----+---------+-------+
| id    | int(11) unsigned zerofill | YES  |     | NULL    |       |
| name  | varchar(20)               | YES  |     | NULL    |       |
+-------+---------------------------+------+-----+---------+-------+
2 rows in set (0.03 sec)
    
mysql> alter table a1 modify id int unsigned;
Query OK, 0 rows affected (0.31 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc a1;
+-------+------------------+------+-----+---------+-------+
| Field | Type             | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+-------+
| id    | int(10) unsigned | YES  |     | NULL    |       |
| name  | varchar(20)      | YES  |     | NULL    |       |
+-------+------------------+------+-----+---------+-------+
2 rows in set (0.06 sec)