These are online, self-guided ways for developers wishing to gain more RDBMS understanding (or have a refresher course) in the two most ubiquitous platforms — MySQL and PostgreSQL.  I add and remove from this list from time to time as material becomes outdated or better bits are found.

Disclaimer:  I didn’t have a hand in creating any of these, but have recommended them in one format or another for several years.

All links open in a new tab.

TUTORIALS

MySQL for Developers
https://www.mysqltutorial.org/

MySQL in 7 Days
https://www.guru99.com/mysql-tutorial.html

Learn MySQL via TutorialsPoint (quite good, complete)
https://www.tutorialspoint.com/mysql/index.htm

Simple SQL Overview (beginners, from W3Schools)
https://www.w3schools.com/sql/

PostgreSQL Tutorial (also has you install it on your platform of choice)
https://www.postgresqltutorial.com/

MAINTENANCE TIPS

15 Useful MySQL/MariaDB Performance Tuning and Optimization Tips
https://www.tecmint.com/mysql-mariadb-performance-tuning-and-optimization/

VIDEO TRAINING

MySQL Tutorial for Beginners [Full Course] (Programming with Mosh)
https://www.youtube.com/watch?v=7S_tz1z_5bA – (3h 10m)

MySQL Tutorial For Beginners (Intellipaat)
https://www.youtube.com/watch?v=WmGgxTpGs_8 – (2h 5m)

Learn PostgreSQL Tutorial – Full Course for Beginners
https://www.youtube.com/watch?v=qw–VYLpxG4 – (4h 20m)

Postgres for Beginners (Simplilearn)
https://www.youtube.com/watch?v=eMIxuk0nOkU – (1h 30m)