We discuss the differences between open-source database systems like MongoDB and relational databases that use SQL. In the early days of open-source, it used to be built to replicate what proprietary software did well. Today, open-source is the innovator because that is what developers expect and need to be successful. In many ways MongoDB builds on the success of relational databases and replicates their best features while allowing for more rapid innovation and time to market. Modern applications deal with data that is both structured and unstructured, which means the database needs to be able to evolve rapidly. At its core, MongoDB is built for developers to improve productivity. It allows them to develop applications much faster and to build what had previously not been possible on relational databases.