Open access relational database I have some very basic knowledge in SQL which I mainly learn from some theoretical data science course. I would like to write a few lines of SQL just to experiment and train myself.

Can you please suggest a few free relational database for which I could write a few queries? Those databases can be about health, economy, climate change, social interaction, genetics, .... whatever seems to be of interest to most people.
https://www.lynda.com/ has free online classes. Some of which include practice databases and datasets to work with. Or you might try Gale. https://education.gale.com/l-slcl/