title: Appending datasets by matched variables
body: <p>I have to append three datasets named <code>A</code>, <code>B</code> and <code>C</code> that contain data for various years (for example, <code>1990, 1991...2014</code>). </p>
  
  <p>The problem is that not all datasets contain all the survey years and therefore the unmatched years need to be dropped manually before appending. </p>
  
  <p>I would like to know if there is any way to append three (or more) datasets that will keep only the matched variables across the datasets (years in this case).</p>