title: Partially distinct select
body: <p>I have a table with fields <code>utm</code> and <code>tracking_id</code>. There may be many <code>tracking_id</code>s for each <code>utm</code>.
  So I need to select distinct utm and any one tracking id for each utm (let`s say thefirst one).</p>
  
  <p>For example, we got the table:</p>
  
  <pre><code>utm   | tracking_id
  -------------------
  ddff  |      1
  ddff  |      2
  llzz  |      3
  ddff  |      4
  ohoh  |      5
  ohoh  |      6
  </code></pre>
  
  <p>And as an output i want to get:</p>
  
  <pre><code>utm   | tracking_id
  -------------------
  ddff  |      1
  llzz  |      3
  ohoh  |      5
  </code></pre>
  
  <p>I use PostgreSQL 9.1.<br>
  Is there a way to do it with SQL?</p>