Best resources to learn the intricacies of relational databases

Courses:

CMU 15-445: Intro to Database Systems

- https://15445.courses.cs.cmu.edu/fall2022/

CMU 15-721: Advanced Database Systems

- https://15721.courses.cs.cmu.edu/spring2020/

CMU 15-799: Self-Driving DBMS

- https://15799.courses.cs.cmu.edu/spring2022/

DBMS specific:

PostgreSQL 14 Internals

- https://postgrespro.com/community/books/internals

Others:

A handy list of distributed system links

- https://pierrezemb.fr/posts/distsys-resources/

I am trying to get a more than superficial level of knowledge on deploying services on relational databases with a lot of traffic. I don't know if it makes sense to grab any manual of a relational database, like PostgreSQL or MySQL, or if that is too specific and it makes more sense to start with a more general book. [Please do not suggest Designing Data Intensive Applications, I am already reading it :) ] Moreover, what are the best resources you know of that you used to learn SQL queries and their syntax properly? Thank you very much!