There are many forms of SQL, but PostgreSQL, Microsoft SQL, and MySQL are the three most widely used. Although each of these forms is an implementation of SQL, their standards were set by different organizations. PostgreSQL and MySQL are both open-source platforms maintained by a wide community of developers, while Microsoft develops Microsoft SQL. Syntactically, they can differ regarding whether they require case sensitivity, whether they use single quotes or double quotes, and what functions they support. As an example, Microsoft SQL server uses GETDATE(), MySQL uses CURDATE(), and PostgreSQL uses CURRENT_DATE(). They also differ in languages supported; MySQL supports languages that PostgreSQL and Microsoft SQL don't, such as Haskell and Ruby. As Microsoft SQL is proprietary, it costs money to license — but that does mean that it comes with robust support.