This is about how I learned it many years back, but it worked well for me.
(1) I was starting with Oracle, so I read through Oracle's Database Concepts (https://docs.oracle.com/cd/B19306_01/server.102/b14220/toc.h...). This contains unnecessarily deep technical detail (which I skimmed), but I learn more easily if I have a strong footing in the concepts so I don't feel disoriented. For example, it was helpful to understand transactions (section 4), undo (section 3), and the data dictionary (section 7; later made it easier to understanding the DDL / DML distinction), consistency (section 13). It also contains an overview of some important SQL concepts in section 24 such as cursors and the fact that there's an optimizer which figures out how to run your query.
(2) I read about database normalization. Understanding how to put things in first, second, and third normal forms was extremely helpful in reshaping how I think of data structures. Learning to use a database is not just about how to write a query. It's also about how to model data. (What does relational, as in relational database, mean anyway?) Even if you're not creating tables, understanding how they are probably organized helps you query them. And once you understand basic modeling stuff like normalization, you can move on to advanced stuff like denormalization. Another part of modeling is keys (natural vs. surrogate/synthetic keys, composite keys, foreign keys, etc.).
(3) I practiced writing SQL queries and learned all the joins, aggregate functions, analytic functions, etc.