Joomla provides two methods to access database: 1. The below method is used to access the Joomla database.

JFactory->getDBO()  (Internal database access)

2. This method is used to access the database other than joomla database i.e, an external database. This might be happen sometimes that you require a totally different database in your project.
You can do this by using the method as shown below-

 JDatabase->getInstance()

Ex:

    $external_db = array(); //prevent problems
     $external_db['driver']   = 'mysql';        // Database driver name
    $external_db['host']     = 'localhost';    // Database host name
    $external_db['user']     = 'root';         // User for database authentication
    $external_db['password'] = 'root';         // Password for database authentication
    $external_db['database'] = 'databasename'; // Database name
    $db = & JDatabase::getInstance( $external_db );

Here, $db is an object of JDatabaseDriver type. Database operations such as insert, update, delete etc., can be done by using the usual methods of joomla.

Ex:

    $external_db = array(); 
     $external_db['driver']   = 'mysql';        // Database driver name
    $external_db['host']     = 'localhost';    // Database host name
    $external_db['user']     = 'root';         // User for database authentication
    $external_db['password'] = 'root';         // Password for database authentication
    $external_db['database'] = 'databasename'; // Database name
    $db = & JDatabase::getInstance( $external_db );
    $sql="select query"; // SQL
    $db->setQuery ( $sql );
   $db->query ();
   $res = $db->loadAssocList ();