How to create and access multiple database in Drupal 7

While developing a website, you may want to use multiple databases. Its very easy in Drupal 7 to use multiple databases. Here are the steps:

1. Edit settings.php in sites/default

$databases = array (
  'default' =>
  array (
    'default' =>
    array (
      'database' => 'drupal7',
      'username' => 'root',
      'password' => '<your-password>',
      'host' => 'localhost',
      'port' => '',
      'driver' => 'mysql',
      'prefix' => '',
    ),
  ),'custom' =>
  array (
    'default' =>
    array (
      'database' => 'custom',
      'username' => 'root',
      'password' => '<your-password>',
      'host' => 'localhost',
      'port' => '',
      'driver' => 'mysql',
      'prefix' => '',
    )
  )
);

2. After making these changes, you can access different database:

db_set_active('custom');
$result = db_query("SELECT nid, title FROM {node} LIMIT 0, 10");
// This result querying from custom database

db_set_active('default');
$result = db_query("SELECT nid, title FROM {node} LIMIT 0, 10");
// This result querying from default database drupal 7