title: How to join Datasets on multiple columns?
body: <p>Given two Spark <strong>Datasets</strong>, A and B I can do a join on single column as follows:</p>
  
  <pre><code>a.joinWith(b, $"a.col" === $"b.col", "left")
  </code></pre>
  
  <p>My question is whether you can do a join using multiple columns.  Essentially the equivalent of the following <strong>DataFrames</strong> api code:</p>
  
  <pre><code>a.join(b, a("col") === b("col") &amp;&amp; a("col2") === b("col2"), "left")
  </code></pre>