title: Python: Joining two dataframes on a primary key
body: <p>I have two DataFrames A and B. I want to replace the rows in A with rows in B where a specific column is equal to each other.</p>
  
  <pre><code>A:
        1           2           3
  0   asd     0.304012       0.358484
  1   fdsa    -0.198157      0.616415
  2   gfd    -0.054764       0.389018
  3   ff         NaN         1.164172
  
  B:
        1           2           3
  0   asd     10.4012       1.458484
  1   fdsa    100.198157      2.015
  </code></pre>
  
  <p>I want the following result:</p>
  
  <pre><code>      1           2           3
  0   asd     10.4012        1.458484   (row merged from B on column 1)
  1   fdsa    100.198157     2.015      (row merged from B on column 1)
  2   gfd    -0.054764       0.389018
  3   ff         NaN         1.164172
  </code></pre>