title: How do I combine_first, indexing specific column two DataFrames into one?
body: <p>After mapping by column c,
      If column A has a value, insert the value of column A; if not, insert column B.</p>
  
  <pre><code>data1                               data2
  
  a    b    c                      a    c    d
  a1   b1   c1                     1a   c1   1d   
       b2   c2                     2a   c2   2d
  a3        c3                     3a   c3   3d
                                   4a   c4   4d
  </code></pre>
  
  <p>The result I want</p>
  
  <pre><code>  result
     a    b   c     
     a1   b1  c1
     2a   b2  c2
     a3       c3
  </code></pre>
  
  <p>I tried the following, but I was not satisfied.</p>
  
  <pre><code>-&gt;&gt;&gt; result = data1.merge(data2, on=['c'])
  Prefixes _x and _y are created. combine_first is not applied.
  
  -&gt;&gt;&gt; result = data1.combine_first(data2)
  It is not mapped by column c.
  </code></pre>
  
  <p>How do I get good results?
  I ask for your help.
  thank you</p>