title: update column with incremental value
body: <p>i have a table like this</p>
  
  <pre><code>userid | points | position
  1      | 100    | NULL
  2      | 89     | NULL
  3      | 107    | NULL
  </code></pre>
  
  <p>i need a query for update the position column ordering by points desc, example result:</p>
  
  <pre><code>userid | points | position
  1      | 100    | 2
  2      | 89     | 3
  3      | 107    | 1
  </code></pre>