title: How to select id with max date group by category in PostgreSQL?
body: <p>For an example, I would like to select id with max date group by category,
  the result is: 7, 2, 6</p>
  
  <pre><code>id  category  date
  1   a         2013-01-01
  2   b         2013-01-03
  3   c         2013-01-02
  4   a         2013-01-02
  5   b         2013-01-02
  6   c         2013-01-03
  7   a         2013-01-03
  8   b         2013-01-01
  9   c         2013-01-01
  </code></pre>
  
  <p>May I know how to do this in PostgreSQL?</p>