Recently I came across an excellent Oracle platform that enables online SQL practice. The platform provides a sort of sandbox where you can create new tables, and use SQL to query those tables. You can use this platform to perform all the exercises listed in the Oracle tutorial .
To get started with the platform, enter the following link,and select Start Coding Now.
Login into the site (or register if you already have an account). You can then write and execute queries by using the Run button at the right side of the window.