Today, my senior recommended me a website to learn sql, http://sqlzoo.net/
This is much better than famous w3schools and better than other interactive SQL sites like http://sqlcourse2.com/groupby.html and http://www.xlinesoft.com/articles/interactive_sql_tutorial/index.htm.
The reason is because you can actually type those query yourself to do those exercises and the result will appear on your right side. If the answer is incorrect, it will gently let you know and answer was provided as well. It is very interactive.
I tried almost all of the tutorial today and learnt much.
Another thing awesome of this learning site is you can compare all major databases’ syntax as well. Example, What is the syntax to view structure of table?. Default is Oracle. But if you want to view SQL Server, you can just click on SQL Server’s tab.