title: Join pandas dataframes based on column values
body: <p>I'm quite new to pandas dataframes, and I'm experiencing some troubles joining two tables.</p>
  
  <p>The first df has just 3 columns:</p>
  
  <pre><code>DF1:
  item_id    position    document_id
  336        1           10
  337        2           10
  338        3           10
  1001       1           11
  1002       2           11
  1003       3           11
  38         10          146
  </code></pre>
  
  <p>And the second has exactly same two columns (and plenty of others):</p>
  
  <pre><code>DF2
  item_id    document_id    col1    col2   col3    ...
  337        10             ...     ...    ...
  1002       11             ...     ...    ...
  1003       11             ...     ...    ...
  </code></pre>
  
  <p>What I need is to perform an operation which, in SQL, would look as follows:    </p>
  
  <pre><code>DF1 join DF2 on 
  DF1.document_id = DF2.document_id
  and
  DF1.item_id = DF2.item_id
  </code></pre>
  
  <p>And, as a result, I want to see DF2, complemented with column 'position':</p>
  
  <pre><code>item_id    document_id    position    col1   col2   col3   ...
  </code></pre>
  
  <p>What is a good way to do this using pandas?</p>
  
  <p>Thank you!</p>