Mechanisms for tracking DB schema changes
What are the best methods for tracking and/or automating DB schema changes? Our team uses Subversion for version control and we've been able to automate some of our tasks this way (pushing builds up to a staging server, deploying tested code to a production server) but we're still doing database updates manually. I would like to find or create a solution that allows us to work efficiently across servers with different environments while continuing to use Subversion as a backend through which code and DB updates are pushed around to various servers.
Many popular software packages include auto-update scripts which detect DB version and apply the necessary changes. Is this the best way to do this even on a larger scale (across multiple projects and sometimes multiple environments and languages)? If so, is there any existing code out there that simplifies the process or is it best just to roll our own solution? Has anyone implemented something similar before and integrated it into Subversion post-commit hooks, or is this a bad idea?
While a solution that supports multiple platforms would be preferable, we definitely need to support the Linux/Apache/MySQL/PHP stack as the majority of our work is on that platform.
Answer
In the Rails world, there's the concept of migrations, scripts in which changes to the database are made in Ruby rather than a database-specific flavour of SQL. Your Ruby migration code ends up being converted into the DDL specific to your current database; this makes switching database platforms very easy.
For every change you make to the database, you write a new migration. Migrations typically have two methods: an "up" method in which the changes are applied and a "down" method in which the changes are undone. A single command brings the database up to date, and can also be used to bring the database to a specific version of the schema. In Rails, migrations are kept in their own directory in the project directory and get checked into version control just like any other project code.
This Oracle guide to Rails migrations covers migrations quite well.
Developers using other languages have looked at migrations and have implemented their own language-specific versions. I know of Ruckusing, a PHP migrations system that is modelled after Rails' migrations; it might be what you're looking for.