I just recently got back into the SQL game, so I’m still in the re-learning process (I haven’t touched SQL in about 10 years).
This is what I am doing.
I started with a “coach” who explained to me what he was doing with the database which is now mine. He answered my questions and provided me with a framework in which to play.
Then I went to look for an on-line tutorial and found this gem: https://www.w3schools.com/sql/default.asp . This is a very well written tutorial for someone who only knows the basics of databases, and has a “try it yourself” functionality so you can actually perform actions to assist your learning.
After the tutorial, it’s a process of experimenting and seeing what happens.
For manipulating the database from outside (i.e. creating test data), I am using Python, because I know Python and sqlite3 (the database I am using) is in the standard library. Thus, the documentation and examples can be found in the Python documentation (for version 2.6 and higher: https://docs.python.org/2/library/sqlite3.html)