title: pandas left join and update existing column
body: <p>I am new to pandas and can't seem to get this to work with merge function:</p>
  
  <pre><code>&gt;&gt;&gt; left       &gt;&gt;&gt; right
     a  b   c       a  c   d 
  0  1  4   9    0  1  7  13
  1  2  5  10    1  2  8  14
  2  3  6  11    2  3  9  15
  3  4  7  12    
  </code></pre>
  
  <p>With a left join on column a, I would like to update common columns BY THE JOINED KEYS. Note last value in column c is from LEFT table since there is no match. </p>
  
  <pre><code>&gt;&gt;&gt; final       
     a  b   c   d 
  0  1  4   7   13
  1  2  5   8   14
  2  3  6   9   15
  3  4  7   12  NAN 
  </code></pre>
  
  <p>How should I do this with Pandas merge function? Thank you.</p>