This is taken from their site: http://sqlfiddle.com/about.html
If you do not know SQL or basic database concepts, this site is not going to be very useful to you. However, if you are a database developer, there are a few different use-cases of SQL Fiddle intended for you:
You want help with a tricky query, and you’d like to post a question to a Q/A site like StackOverflow. Build a representative database (schema and data) and post a link to it in your question. Unique URLs for each database (and each query) will be generated as you use the site; just copy and paste the URL that you want to share, and it will be available for anyone who wants to take a look. They will then be able to use your DDL and your SQL as a starting point for answering your question. When they have something they’d like to share with you, they can then send you a link back to their query.
You want to compare and contrast SQL statements in different database backends. SQLFiddle easily lets you switch which database provider (MySQL, PostgreSQL, MS SQL Server, Oracle, and SQLite) your queries run against. This will allow you to quickly evaluate query porting efforts, or language options available in each environment.
You do not have a particular database platform readily available, but you would like to see what a given query would look like in that environment. Using SQL Fiddle, you don’t need to bother spinning up a whole installation for your evaluation; just create your database and queries here!