Disclaimer: none of these recommendations is sponsored
# sites
https://www.hackerrank.com/domains/sql
https://towardsdatascience.com/10-problems-to-practice-almost-all-sql-concepts-37545e7c5219
https://www.w3resource.com/sql-exercises/
https://www.codecademy.com/resources/blog/sql-code-challenges-for-beginners/
# Book
https://www.amazon.it/SQL-Practice-Problems-learn-doing-ebook/dp/B01N41VQFO