Good courses on database (possibly paid)?

Question

Database

In general, in my work I encounter the fact that I do not know this topic well enough. I can do some simple join. But I want to understand more deeply what and how it works + be able to build an optimal database structure and write complex queries for optimization. I also consider paid courses up to 50k, preferably (since the organization pays for it). Or you can tell me where you took paid courses and it was not a waste of money.
Now I work with MySQL.

Answer 1

Courses are useless, they only benefit their authors. Read tutorials and documentation of your DBMS.

Answer 2

A short course on the Stepika . (there are others free and paid).

Answer 3

Offhand:
https://www.specialist.ru/course/mskvl

Answer 4

There are good MySQL courses here https://shultais.education/courses/sql and here https://stepik.org/course/51562/info