How to pull up knowledge in the database?

Question

MySQL

Working with databases, I periodically come across moments that I either don’t know at all or are very superficial. I want to correct the situation and pump the skill. Advise, please, any literature which will pump over the theory of databases. Interested in both theory in general and specifically Postgresql and MySql. Ideally, I see the presentation from the very basics, smoothly delving into the wilds.

Answer 1

Intuit
Coursera

Answer 2

www.w3schools.com/sql/default.asp

Answer 3

Theory is good, but practice doesn't hurt either: https://pgexercises.com/