title: Pandas analogue of JOIN with WHERE clause
body: <p>I'm doing joining of two dataframe (A and B) in python's pandas. </p>
  
  <p>The goal is to receive all the pure rows from B (sql analogue- right join B on A.client_id=B.client_id where A.client_id is null)</p>
  
  <p>In pandas all I know for this operation is to do merging but I don't know how to set up the conditions (where clause):</p>
  
  <pre><code>x=pd.merge(A,B,how='right',on=['client_id','client_id']
  </code></pre>