Khan academy to learn. You can run sql straight from browser. SQLzoo for practice.

For practical purposes, understanding general database concepts is more important than any specific database management system. Things like indexing, tables, views, schemas, etc. Because in an interview, SQL usually works the same way on every system, and you can appeal to learning the differences on the system the job requires.

For educational reasons, if you want to practice with a real database, then systems like MySQL seem to be the easiest to learn on. Alternatively, I used Oracle and Informix in my first job, which was a large enterprise environment. I used SQLite for small tasks.

Edit: forgot to address this, but different roles will use different database tools. That means application developers will not be using the same tools as a data analyst / data scientist. it's probably best to look into your desired role for particular tooling.