Answer 1

SQL in itself is just the basis for the many dialects implemented in various databases:
https://en.wikibooks.org/wiki/SQL_Dialects_Reference/Introduction#SQL_implementations_covered_in_this_book

So you should try to figure out which dialect you want to learn, based on what job or project you want to pursue. MySQL (and its offshoot MariaDB) is usually a good choice to start with, because that's the most commonly used open source database. I assume that's why SoloLearn focused on MySQL in this course.

Other popular databases: (open source) PostgreSQL, SQLite, (proprietary) Oracle, Microsoft SQL.

Which one fits your goal?