The Detroit chapter recently hosted a beginner level SQL challenge using some exercises from SQL Zoo. That's just one of many free resources available to you if you're trying to learn SQL independently. Check out the list below where I round up a few favorites!
Practice with a web browser IDE
Codecademy - step by step learning and practice
Khan Academy - more learning than practice
SQL Zoo - more practice than learning
Dreamy Cabbage - all practice
FreeCodeCamp.org YouTube beginner course - 4.5 hours, but the video is split into segments, so you can hover over it to find the different topics. This is based on MySQL. Extremely thorough.
FreeCodeCamp.org YouTube Database Design course - 8 hours, more interesting if you're looking to build something
Syntax reference docs
W3 Schools -a beloved classic, TONS of info broken down very minutely. A page for every join. Page formatting leaves something to be desired.
Codecademy cheat sheet - These are very visually pleasing and concise
Mode - Fantastic organization, covers really deep content and it's organized by Beginning, Intermediate, Advanced
DBA diaries - good breakdown of window functions and such
Building SQL challenges - kinda meta, but this is an article about how to create a SQL challenge...if you do build one and put it online, let me know and I will add it to the browser IDE section!
Select Star SQL - a data project about Texas death row inmates that shows you exactly the SQL they used in analyzing the data