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.
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.