title: pandas how to get values from df2 for df1 while df1 and df2 have values overlapped on column(s)
body: <p>I have two <code>df</code>s, the <code>df1</code> looks like,</p>
  
  <pre><code>df1
  id
  1
  2
  3
  4
  5
  
  df2
  doc_no    c_id
  2         22
  3         33
  4         44
  6         66
  7         77
  </code></pre>
  
  <p>values of <code>id</code> in <code>df1</code> overlaps some values of <code>doc_no</code> in <code>df2</code>; I want to get <code>c_id</code> for <code>df1</code> from <code>df2</code> for overlapped <code>doc_no</code>, so the result looks like,</p>
  
  <pre><code>df1
  id    c_id
  1     NaN 
  2     22
  3     33
  4     44
  5     NaN
  </code></pre>
  
  <p>apart from using <code>df1</code> lefts join <code>df2</code> on <code>id</code> and <code>doc_no</code>, I am wondering if there is other better way to do this, which is more concise and efficient. </p>