In this blog, I have listed some of the websites which I used for honing my SQL skills.
One of my favorite one is modeanalytics. The SQL tutorial covers from basic to advanced lessons which I found is very helpful intermediate to experienced SQL persons.
Another website I highly recommended is sqlfiddle. The website provides a very interactive interface. You can create your own schema and tables. It supports most of the RDBMSs such as MS SQL, Oracle, MySQL, PostgreSQL and SQL lite.
sqlzoo provides practice and quizzes to hone your SQL skills. It support only MySQL, Oracle and MS SQL RDBMS engines. This website is good for beginner.
W3schoolsprovides very details tutorial for almost all available RDBMS system. This website is very helpful for beginner to learn all the nuts and bolts of SQL.