Postgresql
createdb example_database
Some platforms, like MySQL support multiple databases per instance, while other platforms like Oracle support one database per instance. You should check the documentation for your particular vendor to see what is supported.
Creating users
Users can be created in most databases using the the CREATE USER statement.
mysql> CREATE USER username;
Specialized Create User Commands
Some relational databases provide additional ways of creating users like specialized command line programs.
Postgresql
createuser username