MySQL in int (m) in M

Http://dev.mysql.com/doc/refman/5.0/en/numeric-type-overview.html

This M is maximum display width. What is maximum display width? See the following example is easy to explain, note 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 (one);
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 (one) 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)

The above example shows that this M represents the width of the display, and he follows Zerofill to make sense. Even if the value of M previously set is smaller than the actual length of the number, it has no effect on the data.