title: How to merge two dataframes and replace the null values with the other's?
body: <p>I have two dataframes that have some columns in common. For example, the first dataframe:</p>
  <pre><code>A    B C D    G
  null 1 2 3    6
  4    5 6 null 7
  </code></pre>
  <p>And the second one:</p>
  <pre><code>A B C D E F
  2 1 2 3 4 5
  4 5 6 8 8 8
  </code></pre>
  <p>How to combine them so that the null values disappear and are replaced by the values from the second dataframe?</p>
  <pre><code>A B C D E F G
  2 1 2 3 4 5 6
  4 5 6 8 8 8 7
  </code></pre>
  <p>I tried using the concat method, but the common columns aren't overlapped, but added to the end of the first dataframe.</p>
  <pre><code>result = pd.concat([df1, df2], axis=1, join='inner')
  </code></pre>