Migrate is a ruby script that uses templates to create implementation independent SQL migration scripts. Migrations are written in straight SQL relevant to each platform, therefore it is programming language independent. The output is an SQL script that can be used to migrate databases.
The intent is not to allow apps to update their own databases on launch since this is not appropriate for most organizations, but rather to provide a script that can be applied to a database during an upgrade.
Check out the sample mysql migrations to get an idea of how to write yours.