https://www.kaggle.com/learn/intro-to-sql
https://www.kaggle.com/learn/advanced-sql
https://www.kaggle.com/datasets?fileType=sqlite
https://www.kaggle.com/datasets?fileType=bigQuery
https://www.databasestar.com/sql-practice/
Keep in mind that every database uses a common core of SQL but they are all using slightly different implementations of SQL. For example, SQLilte and bigQuery and MariaDB are likely to have very similar but slightly different syntax to accomplish the same task.