1、这里的int(11) 与int的大小和存储字节，没有一毛钱关系，int的存储字节是4个字节，最大值为 65536*65536 = 40多亿，对于有符号的int，是20多亿。
2、那么这里的(11) 表示什么意思？
考虑下面的需求，ID字段显示宽度为2，宽度不够的补充0。
3、测试如下：
mysql> desc student;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | 0 | |
| NAME | varchar(16) | YES | MUL | NULL | |
| AGE | int(11) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.01 sec)
mysql> select * from student;
+-----+---------+------+
| id | NAME | AGE |
+-----+---------+------+
| 1 | Andy1 | NULL |
| 11 | Andy11 | NULL |
| 101 | Andy101 | NULL |
+-----+---------+------+
mysql> alter table student modify id int(2);
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc student;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(2) | NO | PRI | 0 | |
| NAME | varchar(16) | YES | MUL | NULL | |
| AGE | int(11) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.01 sec)
mysql> select * from student;
+-----+---------+------+
| id | NAME | AGE |
+-----+---------+------+
| 1 | Andy1 | NULL |
| 11 | Andy11 | NULL |
| 101 | Andy101 | NULL |
+-----+---------+------+
3 rows in set (0.00 sec)
mysql> alter table student modify id int(2) zerofill;
Query OK, 3 rows affected (0.04 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> desc student;
+-------+--------------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+--------------------------+------+-----+---------+-------+
| id | int(2) unsigned zerofill | NO | PRI | 00 | |
| NAME | varchar(16) | YES | MUL | NULL | |
| AGE | int(11) | YES | | NULL | |
+-------+--------------------------+------+-----+---------+-------+
3 rows in set (0.01 sec)
mysql> select * from student;
+-----+---------+------+
| id | NAME | AGE |
+-----+---------+------+
| 01 | Andy1 | NULL |
| 11 | Andy11 | NULL |
| 101 | Andy101 | NULL |
+-----+---------+------+
3 rows in set (0.00 sec)
4、得出结果：
a、int(N) 必须与zerofill 结合，才能有效果。
b、只有宽度不够的，才补充0，宽度超过的，显示实际宽度。
c、使用mysql客户端有效果，使用navicat等第三方软件可能没有效果。