I'm surprised, no one mentioned https://www.sql-ex.ru/ It has few hundred graded SQL exercises (some of them are very hard) + there is a book http://www.sql-tutorial.ru/ that goes over most common errors from submissions. On top of that, there are competition problems with the leaderboard. On top of that, I highly advise everyone to read T-SQL Fundamentals, by Itzik Ben-Gan (and few other books by same author) and do the all exercises in the book.