title: How to add a column to a dataframe from another dataframe with different size
body: <p>I have 2 dataframes and I want to add a new column to one of my dataframes from another dataframe 
  '''</p>
  
  <pre><code>(df1)                            
  Id   height  ...                 
  12   190
  13   180
  34   173
  </code></pre>
  
  <p>.
  .
  .</p>
  
  <pre><code>(df2)
  Id     amount  ...
  12      234
  15      256
  13      248
  </code></pre>
  
  <p>.
  .
  .</p>
  
  <p>'''
  how can I add a column "amount" to df1 that contains the value amount of df2 for each Id of df1. the size of df1 and df2 are not equal</p>