title: Subtract 2 dataframes with different column length if key matches
body: <p>I have a dataframe which has columns with different length. I want to subtract columns VIEWS from each other if the fields URL match.
  This is my code which gives me completely false results and almost exclusively NAN values and floats which both doesn´t make sense to me. Is there a better solution for this or an obvious mistake in my code?</p>
  <pre><code>a = a.loc[:, ['VIEWS', 'URL']]
  b = b.loc[:, ['VIEWS', 'URL']]
  
  df = pd.concat([a,b], ignore_index=True)
  
  df['VIEWS'] = pd.to_numeric(df['VIEWS'], errors='coerce').fillna(0).astype(int)
  df['VIEWS'] = df.groupby(['URL'])['VIEWS'].diff().abs()
  </code></pre>