title: spark scala dataframe merge multiple dataframes
body: <p>I have three files coming in,</p>
  
  <pre><code>## +---+----+----+---+
  ## |pk1|pk2|val1|val2|
  ## +---+----+----+---+
  ## |  1| aa|  ab|  ac|
  ## |  2| bb|  bc|  bd|
  ## +---+----+----+---+
  
  ## +---+----+----+---+
  ## |pk1|pk2|val1|val2|
  ## +---+----+----+---+
  ## |  1| aa|  ab|  ad|
  ## |  2| bb|  bb|  bd|
  ## +---+----+----+---+
  
  ## +---+----+----+---+
  ## |pk1|pk2|val1|val2|
  ## +---+----+----+---+
  ## |  1| aa|  ac|  ad|
  ## |  2| bb|  bc|  bd|
  ## +---+----+----+---+
  </code></pre>
  
  <p>I need to compare the first two files (which I'm reading as dataframe) and identify only the changes and then merge with the third file, so my output should be,</p>
  
  <pre><code>## +---+----+----+---+
  ## |pk1|pk2|val1|val2|
  ## +---+----+----+---+
  ## |  1| aa|  ac|  ad|
  ## |  2| bb|  bb|  bd|
  ## +---+----+----+---+
  </code></pre>
  
  <p>How to pick only the changed columns? and update another dataframe?</p>