title: Cross search two dataframes with pandas
body: <p>I have encountered a problem that I have not seen before.
  The situation is as follows
  I have 2 dataframes somewhat different from each other:</p>
  <ul>
  <li><code>cc_etn</code> has the tickets with refund</li>
  <li><code>etn</code> has the information of the users who have purchased</li>
  </ul>
  <p>I want to search by the id the information of the users who were refunded. What I did was to make a list in <code>cc_etn</code> and so iterate over  <code>etn</code> with the method 'df.loc' to find them, however I get an empty DataFrame.
  The interesting thing here is that to find the user in <code>etn</code>, there are 4 columns that have to be equal to those I have in <code>cc_etn</code>. Now, if I put 4 conditions to <code>df.loc</code> I get empty DataFrame, and if I put a condition if I get the values I want but still missing 3 other columns that would have to be equal to find the user, I also tried to make a chain of <code>df.loc</code> to find them but I also get empty, so my code looks something like this:</p>
  <pre><code>cc_etn = pd.read_csv(CONTRA CARGOS APLICADOS .csv',dtype='str')
  etn = pd.read_csv(query_result_2021-06-29T15_41_38.794753Z.csv')
  
  #here I list the columns that are relevant to finding the user in etn.
  fecha = list(cc_etn['Fecha de Transacción'])
  tarjeta = list(cc_etn['Número de Tarjeta'])
  monto_1 = list(cc_etn['Monto Publicado'])
  codigo = list(cc_etn['Código de \nAutenticación']) 
  
  #here I cycle through to find each user but they all come up empty
  for i in range(len(monto):
    (duplicados.loc[(duplicados['created_at_day'] == fecha[i]) &amp; 
    (duplicados['card_number']==tarjeta[i]) &amp; (duplicados['amount'] == monto[i]) &amp; 
    (duplicados['card_authorization']==codigo[i])]
  
  #now if i print this if it works but i don't get what i want
  duplicados.loc[duplicados['created_at_day'] == fecha[1]
  duplicados.loc[duplicados['card_number']==tarjeta[1]
  duplicados.loc[duplicados['amount'] == monto[1]
  duplicados.loc[duplicados['card_authorization']==codigo[1]
  
  #and if I do this i have again a empty DataFrame
  x = duplicados.loc[duplicados['created_at_day'] == fecha[1]
  y = x.loc[x['card_number']==tarjeta[1]
  </code></pre>
  <p>I have this error too:</p>
  <pre><code>#SettingWithCopyWarning: A value is trying to be set on a copy of a 
  #slice from a DataFrame
  </code></pre>