Any references in particular you recommend to learn schema design, particularly with an orientation towards performance (e.g. for high-volume sites)?
A couple years ago I got dinged on a take-home project that involved building a db schema, but didn't get any specific feedback on my work, and it's sort of haunted me to this day (especially since SQL is one of my primary languages)
Although it is 10 years old, not much has changed in the universe of SQL basics. If I were to capture the essence of good schema design it is mostly about keeping data normalized until you have a really good reason not to. Denormalization is almost always an optimization choice.
And before you optimize you should have basic things covered, like indexes, etc. I have fixed more than one "slow" query by simply adding indicies to everything people are joining on. So, check out a tool like pgAdmin that has a cool query planner optimization feature. What is happening under the hood doesn't matter a /lot/ when learning SQL, but it is really insightful to see how indicies of various types impact performance. I believe this book basically covers it all from a theoretical perspective. Optimization and indices aren't super well covered in SQL for smarties, which make sense, it isn't about optimization but is a little higher level.
There are /tons/ of data sets out there now a days. CSV files, etc. Find some interesting data and start challenging yourself with interesting ways to design that data into a database. I actually design most of my SQL databases using an ORM these days, but, my bedrock knowledge of SQL makes it very efficient and I can avoid committing "SQL sins" (denormalization) prematurely. You will be surprised at how much you can learn on simple data sets :)
Take a look at the "Relational Design Theory"[1] mini-course on Stanford/Lagunitas. Though I had used SQL for years, I took it as part of the early MOOC on databases that was offered in 2011. It is likely to match up with the intuition you already have about what works well in practice, but it can still be a challenge and rewarding to formalize your intuition and understanding.
The way the course is broken up now, you may need some of the other sections [2] like Intro to Relational Databases or Relational Algebra as prerequisites, since I do not remember if it used SQL syntax.
Reminder, Stanford is taking down this course on March 26th, 2020. They let you download courses that are setup for download (this course is not, unfortunately).
A couple years ago I got dinged on a take-home project that involved building a db schema, but didn't get any specific feedback on my work, and it's sort of haunted me to this day (especially since SQL is one of my primary languages)