If you aren't aware of it, there's a free online course on databases (and SQL). I took it back when it ran live, but it's just as good self-paced. Jennifer Widom (Stanford) is an outstanding lecturer and the videos and assignments are excellent. SQLite is used to grade the online exercises, so I'd suggesting installing a local copy to experiment with as it's free. I'd also strongly recommend installing either MySQL or PostgreSQL (I recommend PostgreSQL; both are free) so you can learn while using a full-featured database server. BaseX is very helpful for learning XML and mastering XPath and XQuery for web scraping (also free).
BaseX has a module for handling JSON. I haven't used it personally, but it looks useful for learning about JSON.