title: How to union DataFrames and add only missing rows?
body: <p>I have a dataframe df1, which contains below data:</p>
  
  <pre><code>**customer_id**   **product**   **Val_id**    **rule_name**
       1               A            1               rule1
       2               B            X               rule1
  </code></pre>
  
  <p>I have another dataframe df2, which contains below data:</p>
  
  <pre><code>**customer_id**   **product**   **Val_id**    **rule_name**
       1               A            1               rule2
       2               B            X               rule2
       3               C            y               rule2
  </code></pre>
  
  <p>rule_name values in both dataframes is always fixed</p>
  
  <p>I want a new unionized dataframe df3. It should have all customers from dataframe df1 and all other customers from dataframe df2, which are not present in df1. So final df3 should look like:</p>
  
  <pre><code>**customer_id**   **product**   **Val_id**        **rule_name**
           1               A            1               rule1
           2               B            X               rule1
           3               C            y               rule2
  </code></pre>
  
  <p>Can anyone please help me out to achieve this outcome. Any help will be appreciated.</p>