Database Tab:

Warning: These are very important settings.

The settings displayed in this tab are those that were entered during the Installation procedure for this Joomla! web site.

Warning: These settings are set within the MySQL database itself and are copied into Joomla! to facilitate communication between the web server and the database server. Changes made to the fields in this tab will NOT change the settings in the MySQL database which is administered separately.

If changes are made to these settings in the Global Configuration, and they do not match the MySQL settings themselves, the web site will become inoperable.

Equally, if any of these settings are changed in the MySQL database then those changes MUST also be made in the following fields.

Hostname: This is the name of the host of the database server and, therefore, the database upon which Joomla! is storing its data. For most web developers testing on a local machine, or a dedicated root server localhost is the default name. For those on shared or virtual host servers then this name will probably be different.

MySQL Username: This is the username entered by the MySQL database administrator to access the database.

MySQL Password: This is the password - entered during the installation of Joomla! - that together with the Username above, is used to access the MySQL database upon which Joomla! stores its data. Do not edit this setting unless absolutely necessary.
Warning: For security reasons it is not possible to change the MySQL password from within Joomla!  If the new MySQL username also requires a new password then make sure that you know how to make amendments to your configuration.php file before making the change.  If the MySQL username and password combination are not correct then your Joomla! website will be inoperable until you can amend configuration.php with a valid username and password combination.

MySQL Database: This is the name of the MySQL database upon which Joomla! stores its data.

MySQL Database Prefix: This is the prefix that was entered during the installation of Joomla! It is used for the tables and records in the MySQL database. The default prefix for Joomla! is jos_.

Warning:  Do not change this database prefix setting unless the database tables with this prefix already exist. Failure to heed this warning will result in an inoperable web site.

NOTE: The most common situations where changes to these settings are required is when changing server hosting companies, or transferring a web site and database from a local development server to a live server host.

Check the Joomla! Forums for further information.