The right way to add a column to an existing table in Magento.

upgrade_add_column.php

<?php
$installer = $this->startSetup();

$installer->getConnection()
    ->addColumn(
        $installer->getTable('module/table_alias'),
        'sql_column_name',
        array(
            'type'  => Varien_Db_Ddl_Table::TYPE_INTEGER, // Or any other type from Varien_Db_Ddl_Table
            'nullable' => true, // Or false...
            'default' => null, // Or any default value
            'comment' => 'Column Comment'
        )
    );

$installer->endSetup();
?>