title: How to join datasets with same columns and select one?
body: <p>I have two Spark dataframes which I am joining and selecting afterwards. I want to select a specific column of one of the Dataframes. But the same column name exists in the other one. Therefore I am getting an Exception for ambiguous column.</p>
  
  <p>I have tried this: </p>
  
  <pre><code>d1.as("d1").join(d2.as("d2"), $"d1.id" === $"d2.id", "left").select($"d1.columnName")
  </code></pre>
  
  <p>and this:</p>
  
  <pre><code>d1.join(d2, d1("id") === d2("id"), "left").select($"d1.columnName")
  </code></pre>
  
  <p>but it does not work. </p>