3 Course to start learning Oracle SQL :

Databases for Developers: Foundations

Learn SQL in this FREE 12-part boot camp. It will help you get started with Oracle Database and SQL. – https://devgym.oracle.com/pls/apex/dg/class/databases-for-developers-foundations.html

Databases for Developers: Next Level – A FREE 10-part tutorial to help you improve your Oracle SQL skills. https://devgym.oracle.com/pls/apex/dg/class/databases-for-developers-next-level.html

Analytic SQL for Developers – A FREE 6 module boot camp to help you become an expert with Oracle Analytic SQL functions – https://devgym.oracle.com/pls/apex/dg/class/analytic-sql-for-developers.html

Live SQL tool – https://livesql.oracle.com/apex/f?p=590:1000