The PL/SQL Challenge site by Steven Feuerstein is great for learning various SQL and PL/SQL techniques. I am one of the quiz authors - I write most of the SQL quizzes (and one or two PL/SQL quizzes now and then.)
That means there is now accumulated quite a bit of my work as quizzes - each quiz demonstrating some knowledge of SQL. I could replicate this work as blog posts as well, but instead it is now possible for you to search all my quizzes on PL/SQL Challenge.
I have added that link to the right-hand menu of the blog under "Further SQL Tips" just above "About me". That way anyone interested can easily find more of my SQL work.
I hope it may be useful to you, too. And maybe you find yourself starting to visit the PL/SQL Challenge regurlarly. Even if you aren't very competitive yourself, learning by playing has always proved to be a great way to acquire new knowledge.