SQLZoo
sqlzoo.net
A free interactive SQL tutorial with real database exercises covering SELECT, JOIN, aggregation, subqueries, and window functions — runnable directly in the browser against live datasets.
Why it is useful
One of the most effective ways to learn SQL because every exercise runs immediately against real data, so you see the output of your query instantly. The progression from basic SELECT to complex window functions is well-paced. The Nobel Prize, World, and COVID datasets used in the exercises are interesting enough to keep you engaged. No account required, and the exercises get genuinely challenging.