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 |
+-------------+
| 00000012345 |
+-------------+
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 |
+--------+
|
012345 |
+--------+
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。
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原文：http://www.cnblogs.com/eterwei/p/3753154.html