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First of all we will need a script that allows us to use the Joomla class JDatabase.  To do this we need to define a few constants and then include various library files.  Place code like this in your php script.

<?php
/* Initialize Joomla framework */
define( '_JEXEC', 1 );
define('JPATH_BASE', str_replace('/cron','',dirname(__FILE__)) );
define( 'DS', DIRECTORY_SEPARATOR );
/* Required Files */
require_once ( JPATH_BASE .DS.'includes'.DS.'defines.php' );
require_once ( JPATH_BASE .DS.'includes'.DS.'framework.php' );
/* To use Joomla's Database Class */
require_once ( JPATH_BASE .DS.'libraries'.DS.'joomla'.DS.'factory.php' );
/* Create the Application */
$mainframe =& JFactory::getApplication('site');
/* Create a database object */
$db =& JFactory::getDBO();

Now Joomla is up and running we can begin to use the JDatabase class the same as if we were writing code within a component, module or plugin.  The example I'm going to use is very simple but it will give you an idea of how to work with the database.

/* Create a table called cron with one column `datetime` */
$query = 'CREATE TABLE IF NOT EXISTS '.$db->nameQuote('#__cron').' '
 .'(`datetime` DATETIME) TYPE=MyISAM';
$db->setQuery($query);
$db->query();
/* Die on error */
if($db->getErrorMsg()) { die; }
$query = 'INSERT INTO '.$db->nameQuote('#__cron').' (`datetime`) VALUES ("2011-01-01 23:59:49")';
$db->setQuery($query);
$db->query();
/* Update the cron table with the current date and time */
$query = 'UPDATE '.$db->nameQuote('#__cron').' SET datetime = "'.date('Y-m-d H:i:s',time()).'"';
$db->setQuery($query);
$db->query();
 

This is a very basic script that will create a table called cron and place the current date and time in that table every time it is run.  Admittedly, this is a pointless exercise but the key thing here is looking at how to get a component called JCron to run this script.  You can create your own complex database queries later ;).

Store the entire script somewhere on your site. I created a folder called cron in the main site directory and placed the script there.

Install JCron and enable cron jobs. In the component parameters set the following options:

Next, assuming your script file is called cron.php, you will want to set up the a cron job as follows:

This setup will run the file cron.php every time someone accesses your site.  It will not run if there are no visits to your site.  This is a useful way to set up cron jobs for a novice and a good workaround if you don't have shell access to your server.