int(11) vs. int(anything else)

I'm new to web programming and doing different tutorials that I can find on the net.

I did my research and found out that in int(11), 11 is the maximum display width for integers and it's the default value if unless the integer is UNSIGNED (in this case it's 10).

When I see something like this:

id INT(11) not null AUTO_INCREMENT

I have no questions. But why do I see different things in different tutorials? For examlpe, in some, it says,

id INT(10) not null AUTO_INCREMENT

And even

id INT(4) not null AUTO_INCREMENT

What are the authors of those tuts trying to achieve? None of them seem to bother to give an explanation what 10 or 4 means.

Alright, they're obviously reducing the display width, but why?开发者_StackOverflow What's wrong with the default width of 11? Why would they want to change it? Or are there any other reasons I don't know of?

Thanks.

The x in INT(x) has nothing to do with space requirements or any other performance issues, it's really just the display width. Generally setting the display widths to a reasonable value is mostly useful with the UNSIGNED ZEROFILL option.

//INT(4) UNSIGNED ZEROFILL
0001
0002 
...
0099
...
0999
...
9999
...
10000

//INT(2) UNSIGNED ZEROFILL
01
02 
...
09
...
99
...
100

Without the UNSIGNED ZEROFILL option the value will be left-padded with spaces to the appropriate display width.

//INT(4)
   1
   2 
...
  99
...
 999
...
9999
...
10000

//INT(2)
 1
 2 
...
 9
...
99
...
100

The number is just a representational option called "display width". It may be used by some applications to pad the field when displaying numeric datatypes.

The int size is neither bits nor bytes. It's just the display width, that is used when the field has ZEROFILL specified.

This blog explains the meaning of int(size) in MySQL.

From the docs:

"This optional display width may be used by applications to display integer values having a width less than the width specified for the column by left-padding them with spaces. (That is, this width is present in the metadata returned with result sets. Whether it is used or not is up to the application.)" That is, int(4) is politely asking that 123 be displayed as " 123". Don't know how many things actually pay attention or why they'd bother in a tutorial.

"The display width does not constrain the range of values that can be stored in the column. Nor does it prevent values wider than the column display width from being displayed correctly." So if you shove 123456 into an int(4) it will still be 123456.

Nothing to do with disk space or performance, just a hint to the application retrieving the data that it would like to be displayed with at least N digits.

The related ZEROFILL option can be used in conjunction to actually pad out the returned data. So 123 in an int(4) ZEROFILL is returned as 0123.

It's not performance increase neither difference in maximum allowed size.

It's only used when ZEROFILL is applied on that column. (See: http://alexander.kirk.at/2007/08/24/what-does-size-in-intsize-of-mysql-mean/)

Yes it specifies the display width, and this comes into play only when ZEROFILL is specified. So at that time it can pad the field value with the required number of zeros.

I agree that the MySQL manual is a little vague on the length/display width of integers. You might think that this limits the value you can store in the column to the number of digits but the valid range of the colomn doesnt change, wether you set it to 1 or 11.

What it really does is determine the display width of the column's value. The only case this is usefull is if you use ZEROFILL to pad your values with zero's.

So basically there is no real difference between INT(1) and INT(11) in terms of what you can store in the column, the only case when it becomes relevant is when you want to ZEROFILL your values.

More info:
http://alexander.kirk.at/2007/08/24/what-does-size-in-intsize-of-mysql-mean/
http://dev.mysql.com/doc/refman/5.0/en/numeric-types.html

Copying from this blog article:

MySQL has a little know feature for numerical types known as zerofill. This feature effects the display size of numerical types. Unlike the string types the number inside the parentheses is not the storage size in characters for the type. For numerical types the type name itself determines storage size.

The integer type it’s the padding size for zerofill.