title: Joining two datasets in spark sql (java)
body: <p>I have 2 <code>Dataset</code>s with 4 columns each.<br></p>
  
  <p>My dataSets:<br></p>
  
  <pre><code>'left':
  a | b | c | d 
  1 | 2 | 3 | 4
  
  'right':
  a | b | c | e 
  1 | 2 | 3 | 5
  </code></pre>
  
  <p>What i would like to achieve:<br></p>
  
  <pre><code>a | b | c | d | e 
  1 | 2 | 3 | 4 | 5
  </code></pre>
  
  <p>My code:<br></p>
  
  <pre><code>left.join(right,left.col(a).equalTo(right.col(a))
                          .and(left.col(b).equalTo(right.col(b)))
                          .and(left.col(c).equalTo(right.col(c)))
                          )),"left");
  </code></pre>
  
  <p>I would like to add column 'e' from <code>right</code> to <code>left</code>, but what i get in return is:<br></p>
  
  <pre><code>a | b | c | d | a | b | c | e |
  </code></pre>
  
  <p>I get only a concatenation of the headers.<br>
  I thought from a logical point of view my query is correct, i am apparently failing at asking Spark how to perform the join.<br>
  Any tip?</p>