title: Only copy one key-column into merged DataFrame
body: <p>Consider the following DataFrames:</p>
  
  <pre><code>df1 = pd.DataFrame({'a': [0, 1, 2, 3], 'b': list('abcd')})
  df2 = pd.DataFrame({'c': list('abcd'), 'd': 'Alex'})
  </code></pre>
  
  <p>In this instance, <code>df1['b']</code> and <code>df2['c']</code> are the key columns. So when merging:</p>
  
  <pre><code>df1.merge(df2, left_on='b', right_on='c')
     a  b  c     d
  0  0  a  a  Alex
  1  1  b  b  Alex
  2  2  c  c  Alex
  3  3  d  d  Alex
  </code></pre>
  
  <p>I end up with both key columns in the resultant DataFrame when I only need one. I've been using:</p>
  
  <pre><code>df1.merge(df2, left_on='b', right_on='c').drop('c', axis='columns')
  </code></pre>
  
  <p>Is there a way to only keep one key column?</p>