title: Joining two dataframes on unique ID, but using another value if id doesn't exist
body: <p>I have two dataframes as such:</p>
  
  <pre><code>UID    mainColumn .... (other columns of data)
  1      apple
  2      orange
  3      apple
  4      orange
  5      berry
  ....
  
  UID2   mainColumn2
  1      truck
  3      car
  4      boat
  5      plane
  ...
  </code></pre>
  
  <p>I need to join the second dataframe onto dataframe based on UID, however if df2 does not contain a uid, then the maincolumn value is the one I'd to use. In the above example, UID2 does not contain the value 2, so the final table would look something like</p>
  
  <pre><code>UID    mainColumn ....
  1      truck
  2      orange
  3      car
  4      boat
  5      plane
  ...
  </code></pre>
  
  <p>Now I'm aware we can do something in the form of</p>
  
  <pre><code>df1=df1.merge(df2,left_on='UID', right_on='UID2')
  </code></pre>
  
  <p>But the issue I have is not replacing the missing values, and making sure they are still included. Thanks!</p>