The following list of sites give users the chance to practice writing SQL through a series of progressive exercises. These are the sites I’ve found through initial research, but is not necessarily an exhaustive list. My plan is to add additional content to this resource in time.
https://mystery.knightlab.com/ – A murder mystery you can solve using SQL
https://sqlzoo.net/ – Learn SQL by writing queries against a variety of datasets
http://www.sqlishard.com/ – Guided SQL exercises on a SQL Server database to learn T-SQL
https://sqlbolt.com/ – 18 progressive exercises
https://pgexercises.com/ – Learn SQL against a PostgreSQL-based database
https://www.sololearn.com/Course/SQL/ (requires registration)