title: Remove duplicates based on multiple columns
body: <p>I've got a multiple fields Primary Key that consists of (col1,col2,col3). 
  However, before the PK was added, there have been a lot of duplicates added which need to be removed.</p>
  
  <p>So for my table if I have: </p>
  
  <pre><code>Col1 Col2 Col3
  ---- ---- ----
    2    3   3
    2    3   3
    2    3   4
    2    4  null
    2    4  null
  </code></pre>
  
  <p>the query should result in </p>
  
  <pre><code> Col1 Col2 Col3
   ---- ---- ----
    2    3   3
    2    3   4
    2    4  null
  </code></pre>
  
  <p>Only 3rd column is nullable.</p>