http://everythingmysql.ning.com/profiles/blogs/data-type-confusion-what-is-an
Over and over I see customers that don‘t understand what int(11) really means. Their confusion is understandable. Many know what defining a char(10) means (a fixed-sized character string that allows up to 10 characters). However, ints are different.
First of all, there are 5 types of integer. They are all fixed size.
| Type | # of bytes |
| tinyint | 1 |
| smallint | 2 |
| mediumint | 3 |
| int | 4 |
| bigint | 8 |
As you can see from the chart, an int is always 4 bytes. That can store signed numbers from -2 billion to +2 billion (and unsigned numbers 0 to 4B). So, what does it mean if you declare an int(5)? It does not restrict the number of digits to 5... It may actually do nothing! The (5) part is a display width. It‘s only used if you use UNSIGNED and ZEROFILL with an integer type. Then the display of those numbers will be zero-padded on the left to 5 digits if they contain less than 5 digits. Example:
CREATE TABLE `foo` (
`bar` int(5) unsigned zerofill DEFAULT NULL
)
SELECT * FROM foo; +---------+
| bar |
+---------+
| 00042 |
| 00101 |
| 9876543 |
+---------+
Data type confusion: what is an int(11)?
原文：http://www.cnblogs.com/kungfupanda/p/5927505.html