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I asked Janelle Coburn via tweet about how to learn some of the intermediate topics in SQL.
Here reply can be found here:
SQL Shack has some good stuff, I feel like so much of what I've learned is through trial and error (since I'm not a DBA but I've played one on TV sometimes). I would recommend checking out indexes, and query plans.
One last resource - I have seen Erland speak and I particularly like this article (but he has more)