So, you got the error WD php: Exception: To start over, you must empty your existing database. when attempting a drush si?

But you're a professional developer, how is this possible?

After you have checked that php5-mysql is installed and your settings.php points to a database that exists, try pointing your host to your local ip rather than localhost:

$databases['default']['default'] = array(
  'driver' => 'mysql',
   'database' => 'your_database',
   'username' => 'your_username',
   'password' => 'your_password',
   'host' => '',
   'prefix' => '',

I ran into this when automating a test built on circleci. And this resolved it:

~~~php 'host' => '', ~~~

Hope it saves some developer a bruised forehead and an identity crisis.