SQL Fiddle is a free website (sqlfiddle.com), which allows you to create a relational database schema and insert data into tables online. After creating a DB schema and data, you can run SQL queries online using the data. SQL Fiddle supports different database providers, which include Oracle, MySQL, and PostgreSQL.
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.
Visit sqlfiddle.com