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。
转载于:https://www.cnblogs.com/eterwei/p/3753154.html
