https://lagunita.stanford.edu/courses/DB/2014/SelfPaced/abou...
After this I would have a look at the postgres documentation. There's so many great examples between there and stack overflow.
I really hate knowing how easy it is to do some queries and struggling to use the Django ORM to achieve what I could write in minutes - but for maintainability I hate dropping down to SQL in that context.
For a fun advanced tutorial, have a go at pivot tables with crosstab using the docs. I've used it for analytics queries to chart categories as headings, with complex aggregations etc.
https://www.postgresql.org/docs/9.1/tablefunc.html
Window functions are another classic
https://www.postgresql.org/docs/9.1/tutorial-window.html
In general working with SQL and gradually solving the problems you have learning as you go is excellent. If you want faster than that there probably are courses but I really do rate their docs. Postgres is great.
It includes short videos, quizzes, articles, and exercises.