Khan Academy has a a number of good resources you can use to learn SQL.
For a quick overview, check out their Hour of Code for SQL
Once you’ve gotten through that you can learn more about SQL in their in depth tutorial
https://www.khanacademy.org/computing/computer-programming/sql
Codecademy recently created an SQL course. I’ve yet to try it but they tend to be great because you can write code and get immediate feedback in an console that is hosted in your web browser. Their other courses on python and javascript have been really good!
https://www.codeschool.com/courses/try-sql is also a decent intro.
This site was super helpful: http://studybyyourself.com/seminar/sql/course/?lang=en
Some of the best SQL tutorials to help you learn SQL.