I learned SQL the hard way. Some basic MySQL and design in college, a little MS Access on the side, and TSQL to help pay the bills. Between then and now, I can't decide whether I've learned a great deal more or only improved incrementally.
I attended a college class, read books, watched live webcasts like SQLLunch and 24 Hours of PASS, attended SQLSaturday events, and learned from far more knowledgeable friends.
All of this helped, but when someone asks me how to learn SQL, I still don't have a good answer.
Of all of the many places I learned from, what made it all stick and make sense was putting hands-on-keyboard and using all the new things I was learning. Practice problems, programming challenges, and real production problems made all of those lessons real.
The goal of this site is to provide a guided, hands-on experience. A way to learn the basics or advance our skills through actual hands-on-keyboard experience. A mix of guidance and experimentation with a real SQL database evaluating our progression.
I believe strongly in personal skill development and have always wished there was something like this being actively developed. A site I could point developers to when they're trying to advance their SQL skills, somewhere more advanced users can try things like windowing functions with some guidance and clear explanations. A hands=-on resource to complement the videos, books, blogs and such.
For now I'm paying for this site out of pocket and working on it in my spare time. I provide a lot of information on the technology and will probably open up the source later on as well.