PostgreSQL Selecting last updated of category type

I have a table cars, which has fields make model and update_at.

updated_at is a DateTime

make    | model     | updated_at
----------------------------------
Ford    | Fiesta    | 12-Jan-2010
Chevy   | Sliverado | 13-Jan-2010
Ford    | Focus    开发者_StackOverflow中文版 | 13-Jan-2010

I want to find the last updated car of each make like:

make    | model     | updated_at
---------------------------------
Chevy   | Sliverado | 13-Jan-2010
Ford    | Focus     | 13-Jan-2010

Normally I would just group by but in postgresql if I group by make I have to include any other selected columns in the group by or in an aggregate function.

How do I get my desired result.

---

PostgreSQL 8.4+:

WITH sample AS (
   SELECT t.make,
          t.model,
          t.updated_at,
          ROW_NUMBER() OVER(PARTITION BY t.make
                                ORDER BY t.updated_at DESC) AS rank
     FROM CARS t)
SELECT s.make,
       s.model,
       s.updated_at
  FROM sample s
 WHERE s.rank = 1

PostgreSQL prior to 8.4:

This uses a self join, but the problem is you'll get duplicates if a make has two or more models with the same maximum updated_at value:

SELECT x.make,
       x.model,
       x.updated_at
  FROM CARS x
  JOIN (SELECT t.make,
               MAX(t.updated_at) AS max_date
          FROM CARS t
      GROUP BY t.make) y ON y.make = x.make
                        AND y.max_date = x.updated_at