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 I add and remove from this list from time to time as material becomes outdated or better bits are found.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.
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MySQL for Developers
MySQL in 7 Days
Learn MySQL via TutorialsPoint (quite good, complete)
Simple SQL Overview (beginners, from W3Schools)
PostgreSQL Tutorial (also has you install it on your platform of choice)
15 Useful MySQL/MariaDB Performance Tuning and Optimization Tips
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)