Tobias Petry shows off a feature in Postgres:
SQL is a straightforward and expressive language, but it sometimes lacks constructs for writing queries in a simple way. It’s more complicated than it should be to write a query to get, e.g., only the most expensive order for every customer of the current fiscal year. You can either use PostgreSQL’s vendor-specific DISTINCT ON feature or window functions for every other database.
Click through for the normal method (which works for MySQL, Oracle, SQL Server, etc.) and the Postgres way.