Wow. Sun just bought MySQL. I wonder if/how this will change their offerings? Back when I had occasion to mess with such DBs, I was partial to PostgreSQL anyway; MySQL was kind of toyish as DBs go at the time. It didn't even have subselects, and was not even fully ACID compliant, making it great to back a website, but not anything putting a premium on transactional integrity or needing more sophisticated SQL constructs. This has been addressed in more recent years, and maybe Sun's new ownership will result in MySQL moving upmarket (at least in a paid variant, perhaps).
More detail in this MySQL AB blog entry.