You don't really need certs for learning SQL. If you already have the aptitude from creative problem solving with data (especially within Excel), SQL should be pretty simple for you to learn.
I like W3 Schools because it offers a sandbox with multiple tables for you to practice on. Once you get a pretty basic grasp of the commands, you can just "play" around in the sandbox and practice more intricate queries such as joins.
https://www.w3schools.com/sql/