title: Efficient way to select all values from one column not in another column
body: <p>I need to return all values from <code>colA</code> that are not in <code>colB</code> from <code>mytable</code>. I am using:</p>
  
  <pre><code>SELECT DISTINCT(colA) FROM mytable WHERE colA NOT IN (SELECT colB FROM mytable)
  </code></pre>
  
  <p>It is working however the query is taking an excessively long time to complete.</p>
  
  <p>Is there a more efficient way to do this?</p>