Thanks for the question. MySQL actually hasn't been the default for a while, I think back before Rails 2.0. The default database for testing and development now

Thanks for the question. MySQL actually hasn't been the default for a while, I think back before Rails 2.0. The default database for testing and development now is SQLite; in production, the tutorial uses PostgreSQL at Heroku. I recommend eventually learning how to run Postgres locally as well, but when you're just starting out SQLite is much easier, requiring essentially zero setup.