Sometimes we are required to add a custom field in our database table for our extension in magento. For this purpose we will perform it with the installer. But before moving we must have concept of creating a new module in magento, considering this point lets check out the steps in brief 

1. First create a module in our case we have Madison(Namespace) and Island(module) 
2. Then create two folders etc and sql.
3. create a config.xml in etc.
4. create a folder madison_island_setup in sql.
5. make file name mysql4-install-0.1.0.php in sql folder.
6. Create a xml file i.e., Madison_Island.xml in app/etc/modules.

Now lets see in details actually what we have to do,

After creating the config.xml write the below code in it :

<?xml version="1.0"?>
<config>
    <modules>
        <Madison_Island>
            <version>0.1.0</version>
        </Madison_Island>
    </modules>

    <global>

<fieldsets>
                    <sales_convert_quote_item>
                        <itemcomment>
                            <to_order_item>*</to_order_item>
                        </itemcomment>
                    </sales_convert_quote_item>
</fieldsets>

    <models>
        <island>
            <class>Madison_Island_Model</class> <!-- Model class files -->     
            <resourceModel>island_mysql4</resourceModel> <!--Resource model -->
        </island>
        <island_mysql4>
            <class>Madison_Island_Model_Mysql4</class>
        </island_mysql4>
    </models>
    <resources>  
        <madison_island_setup>
            <setup>
                <module>Madison_Island</module>
            </setup>
            <connection>
                <use>core_setup</use>
            </connection>
        </madison_island_setup>
        <madison_island_write>
            <connection>
                <use>core_write</use>
            </connection>
        </madison_island_write>
        <madison_island_read>
            <connection>
                <use>core_read</use>
            </connection>
        </madison_island_read>
    </resources>

  </global> 

</config>

In the above our code resides in between <models></models>. Here, we have defined the  model class file and resource model. Also the <version>0.1.0</version> tell the version of our extension. we must have our installer file name matched with this version as our installer file has(mysql4-install-0.1.0.php).

Now in the mysql4-install-0.1.0.php  write the below code :

<?php

$installer = $this;
$installer->startSetup();
$installer->run("
	ALTER TABLE {$this->getTable('sales_flat_quote_item')}
	ADD COLUMN `itemcomment` VARCHAR(45) NOT NULL;
	");
$installer->endSetup();

In this file we are executing the Alter query on the table 'sales_flat_quote_item' to add a new column 'Itemcomment'.

Now to tell the magento we have created a module and its active make a file Madison_Island.xml with the below code in  app/etc/modules:

<?xml version="1.0"?>
<config>
    <modules>
        <Madison_Island>
            <active>true</active>
            <codePool>local</codePool>
        </Madison_Island>
    </modules>
</config> 

The Installer gets executed when we hit our magento site url for the first time and add the field is added in the database.