How do I choose a database?
Well choosing a database for your site depends on many ways,
I prefer MySQL to keep it quite simple.

MySQL is preferable PHP sites because:


Drupal 7 now comes with default table engine which is InnoDB(advantages over MyISAM) for
MySQL
MySQL has choice of table types for better read performance.
Drupal has PHPMyAdmin Web Interface for MySQL.
WAMP,XAMP comes with MySQL so it makes life easier when you use MySQL
Drupal's much better tested with MySQL


http://drupal.org/node/37268