title: How can I add a column to this union result?
body: <p>I have this query (which I removed some keys from for brevity's sake): </p>
  
  <pre><code>SELECT id as in_id, out_id, recipient, sender, read_flag 
    FROM received WHERE recipient=1
  UNION ALL 
  SELECT in_id, id AS out_id, recipient, sender, read_flag  
    FROM sent WHERE sender=1 
  </code></pre>
  
  <p>Which combines the results from two tables showing messages sent and received by a given user. What I'd like to do is add a column/flag to the result to distinguish which table the row belongs to so when I display them I can show a relevant icon for sent or received messages. How would I add this?  </p>