title: Join DataFrames, but keep only columns of one
body: <p>I have a <code>DataFrame</code> <code>df1</code>:</p>
  
  <pre><code>| ID | A |
  ---------
  | 1  | 4 |
  | 1  | 4 |
  | 2  | 1 |
  | 2  | 3 |
  | 3  | 2 |
  </code></pre>
  
  <p>and a <code>DataFrame</code> <code>df2</code>:</p>
  
  <pre><code>| ID | B |
  ---------
  | 1  | 2 |
  | 2  | 2 |
  | 3  | 9 |
  </code></pre>
  
  <p>I want to (left) join these, but only want to keep the columns of <code>df2</code>.
  What is a short and easy solution for this?</p>
  
  <p>The resulting <code>DataFrame</code> should look somehow like this:</p>
  
  <pre><code>|ID | B |
  ---------
  | 1 | 2 |
  | 1 | 2 |
  | 2 | 2 |
  | 2 | 2 |
  | 3 | 9 |
  </code></pre>