http://dev.mysql.com/doc/refman/5.0/en/numeric-type-overview.html
这个M 就是maximum display width。那什么是maximum display width?看了下面的例子很容易说明了，注意zerofill ：
mysql> create table b ( b int (4));
Query OK, 0 rows affected (0.25 sec)
mysql> insert into b values (
12345
);
Query OK, 1 row affected (0.00 sec)
mysql> select * from b;
+-------+
| b |
+-------+
| 12345 |
+-------+
1 row in set (0.00 sec)
mysql> alter table b change b b int(11);
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> select * from b;
+-------+
| b |
+-------+
|
12345
|
+-------+
1 row in set (0.00 sec)
mysql> alter table b change b b int(11) zerofill
;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> select * from b
;
+-------------+
| b |
+-------------+
| 000000
12345
|
+-------------+
1 row in set (0.00 sec)
mysql> alter table b change b b int(4) zerofill
;
Query OK, 1 row affected (0.08 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> select * from b
;
+-------+
| b |
+-------+
| 10000 |
+-------+
1 row in set (0.00 sec)
mysql> alter table b change b b int(6) zerofill
;
Query OK, 1 row affected (0.01 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> select * from b;
+--------+
| b |
+--------+
| 0
12345
|
+--------+
1 row in set (0.00 sec)
以上的例子说明了，这个M 的表示显示宽度，他跟着zerofill 一起才有意义。就算前面设置的M的值比数值实际的长度小对数据也没有任何影响。