title: Python update table's value using another table's values
body: <p>I use python and pandas.</p>
  
  <p>I want to make like a [df_result].
  df1's and df2's a,b is condition.
  if a,b is equal value, change the df1's 'd' column value by df2's 'd' column value.</p>
  
  <p>how to make it?
  I don't know any solution..</p>
  
  <p><strong>df1</strong></p>
  
  <pre><code>a b c d
  
  1 2 5 1
  
  1 5 5 1
  
  2 3 4 1
  </code></pre>
  
  <p><strong>df2</strong></p>
  
  <pre><code>a b d
  
  1 2 2
  
  1 2 2
  
  2 3 4
  </code></pre>
  
  <p><strong>df_result</strong></p>
  
  <pre><code>a b c d
  
  1 2 5 2
  
  1 5 5 1
  
  2 3 4 4
  </code></pre>