title: joining two dataframes having duplicate row
body: <p>I have the following two dataframes</p>
  
  <p>df1</p>
  
  <pre><code>+--------+-----------------------------
  |id  | amount       | fee             | 
  |1   | 10.00        | 5.0             |
  |3   | 90           | 130.0           |
  </code></pre>
  
  <p>df2</p>
  
  <pre><code>+--------+--------------------------------
  |exId  | exAmount     | exFee            | 
  |1     | 10.00        | 5.0              |
  |1     | 10.0         | 5.0              |
  |3     | 90.0         | 130.0              |
  </code></pre>
  
  <p>I am joining between them using all three columns and trying to identify columns which are common between the two dataframes and the ones which are not. </p>
  
  <p>I'm looking for output:</p>
  
  <pre><code>+--------+--------------------------------------------
  |id  | amount       | fee  |exId  | exAmount | exFee | 
  |1   | 10.00        | 5.0  |1     | 10.0     | 5.0   |
  |null| null         | null |1     | 10.0     | 5.0   |
  |3   | 90           | 130.0|3     | 90.0     | 130.0 |
  </code></pre>
  
  <p>Basically want the duplicate row in df2 with exId 1 to be listed separately.
  Any thoughts?</p>