title: Append based on condition using pandas
body: <p>I have a data frame <code>df1</code> with columns <code>ID2, Name2, ID3, Name3</code>. I have other two data frame <code>df2</code> (columns: <code>Name2, ID2</code>) and <code>df3</code> (columns: <code>Name3, ID3</code>) with the same columns. I am trying to merge <code>df2</code> and <code>df3</code> based on matching ID, which I do as follows:</p>
  
  <pre><code>Solution = pd.merge(df2,df3, left_on='ID2',right_on='ID3').sort_values('Name2')
  </code></pre>
  
  <p>Now I want to append this result to <code>df1</code>, only if ID2/ID3 doesn't already exist in <code>df1</code>. How do I do that?</p>