title: Spark: Joining Dataframes
body: <p>I need to join two DataFrames as follows:</p>
  
  <ol>
  <li>If keys match, get values from Right side.</li>
  <li>If key on the left side doesn't exist on Right. Take values from left.</li>
  <li>If key on the right side doesn't exist on Left. Take values from right.</li>
  </ol>
  
  <p>What's the best way to do this using DataFrames? Previously I used to do this using 'cogroup' method of RDD, which is not available on DataFrames. </p>