What is a easy project to learn MySQL?
SQL as a language has a pretty uncomplicated syntax. The real complexity of problems lies in the database design, database normalization and database optimization.
I do not know if that is the best way to learn MySQL but you can try to design and create a database for, let’s say, an online shop that offers t-shirts. Remember that you have to store information about users (their passwords must be stored in a secure way), products, categories of products, orders etc.
In the next step you should read a bit about database normalization and try to correct mistakes made in the previous step.
In the last step think about what kind od queries would be executed most times and try to optimize your database to execute these queries as fast as possible.
But the true differences between database engines are mostly related to the managing databases (for example how we scale databases), but the best way to lear about that is to simply encounter the need to do that and do that.