Can anyone recommend an online SQL course that would benefit a Rails dev?

I'm looking to build on my very limited SQL knowledge and do an online course. A lot of course have an emphasis on certain topics, like data analysis or MySQL, for example. I'm using mostly SQLite in development and Postgres in production (as I deploy to Heroku mostly). Does anyone have a course that they could recommend? I'm happy to pay a little for a Udemy or Coursera course or something similar.