There are a lot of free online resources for learning SQL, but be aware that there are significant differences between the various flavors of SQL: MySQL, Postgresql, Microsoft SQL, SQLite...
Knowing one will help with the others, but is by no means fully interchangeable.
So the real question: what are you hoping to accomplish with SQL, and for what environment?
Many websites use MySQL/MariaDB on the back end, in part because it's open source and free to use.
Many corporations with Windows networks use Microsoft SQL on the back end
Huge companies may use Oracle, but you won't see that much in smaller companies due to the $$$ licensing costs
Small applications may use SQLite, which is free and self-contained.
Combined "all of the above" SQL courses are likely going to be extremely generic and not give you a ton more than spending an hour on Google will tell you as well.
Newbie
