www.sql-ex.ru + the book on that site are by far the best SQL exercises I have seen. PS. I know you asking about just RAW SQL, but I highly suggest you get into

www.sql-ex.ru + the book on that site are by far the best SQL exercises I have seen. PS. I know you asking about just RAW SQL, but I highly suggest you get into understanding how particular DBMS works in depth, what plans it builds, what concurrency mechanisms it uses. Writing raw SQL will get you only so far, making those queries actually production ready is completely different ballgame and for that, you don't need to be super proficient in SQL.