Data type confusion: what is an int(11)?

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 | 
+---------+