If you also struggled to find open source database, JUST to practice simply queries link below will come you like light at the end of the tunnel. At least in my case was. After many tries to find open MySQL databases, ultimately I gave up. Luckily, I came across to this link somehow. And it helped me hugely, not only for making queries and joinings, but also from practising machine learning algorithms on relational datasets.
I hope this will help you too https://relational.fit.cvut.cz/search,
Enjoy!