Mechanisms for tracking DB schema changes
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.
We use something similar to bcwoord to keep our database schemata synchronized across 5 different installations (production, staging and a few development installations), and backed up in version control, and it works pretty well. I'll elaborate a bit:
To synchronize the database structure, we have a single script, update.php, and a number of files numbered 1.sql, 2.sql, 3.sql, etc. The script uses one extra table to store the current version number of the database. The N.sql files are crafted by hand, to go from version (N-1) to version N of the database.
They can be used to add tables, add columns, migrate data from an old to a new column format then drop the column, insert "master" data rows such as user types, etc. Basically, it can do anything, and with proper data migration scripts you'll never lose data.
The update script works like this:
- Connect to the database.
- Make a backup of the current database (because stuff will go wrong) [mysqldump].
- Create bookkeeping table (called _meta) if it doesn't exist.
- Read current VERSION from _meta table. Assume 0 if not found.
- For all .sql files numbered higher than VERSION, execute them in order
- If one of the files produced an error: roll back to the backup
- Otherwise, update the version in the bookkeeping table to the highest .sql file executed.
Everything goes into source control, and every installation has a script to update to the latest version with a single script execution (calling update.php with the proper database password etc.). We SVN update staging and production environments via a script that automatically calls the database update script, so a code update comes with the necessary database updates.
We can also use the same script to recreate the entire database from scratch; we just drop and recreate the database, then run the script which will completely repopulate the database. We can also use the script to populate an empty database for automated testing.
It took only a few hours to set up this system, it's conceptually simple and everyone gets the version numbering scheme, and it has been invaluable in having the ability to move forward and evolving the database design, without having to communicate or manually execute the modifications on all databases.
Beware when pasting queries from phpMyAdmin though! Those generated queries usually include the database name, which you definitely don't want since it will break your scripts! Something like CREATE TABLE mydb.newtable(...) will fail if the database on the system is not called mydb. We created a pre-comment SVN hook that will disallow .sql files containing the mydb string, which is a sure sign that someone copy/pasted from phpMyAdmin without proper checking.
My team scripts out all database changes, and commits those scripts to SVN, along with each release of the application. This allows for incremental changes of the database, without losing any data.
To go from one release to the next, you just need to run the set of change scripts, and your database is up-to-date, and you've still got all your data. It may not be the easiest method, but it definitely is effective.
The issue here is really making it easy for developers to script their own local changes into source control to share with the team. I've faced this problem for many years, and was inspired by the functionality of Visual Studio for Database professionals. If you want an open-source tool with the same features, try this: http://dbsourcetools.codeplex.com/ Have fun, - Nathan.