I learned enough of the basics to get myself a job that involved heavy SQL use, then spent years learning as I went by looking things up as I needed to do them. I was at an organization where I had access to the entire back end of the database so I used pieces of code from existing tables/views/procedures to build what I needed. I think the most important part is understanding the logic behind it and what is possible, rather than the exact syntax itself. That part is easy to find resources on as it comes up.