title: UNION ALL tables with different column names
body: <p>I have two tables that are similar and I am trying to join them in a Union and then probably group by.  I am looking to add a column of null or 0 where the tables do not overlap.  </p>
  
  <pre><code>SELECT count(traffic_volume_1) as traffic_volume_1, 
         traffic_source, 
         timestamp
  FROM table_1
  UNION ALL 
         count(traffic_volume_2) as traffic_volume_2, 
         traffic_source, 
         timestamp
  FROM table_2
  ...?
  </code></pre>
  
  <p>I am looking for a return that would look like: </p>
  
  <pre><code>traffic_volume_1, traffic_volume_2, timestamp, traffic_source
  77777           , 0               , 2018-02-09, US
  0               , 928320          , 2018-02-09, EU
  </code></pre>
  
  <p>Any ideas? </p>