title: Select columnValue if the column exists otherwise null
body: <p>I'm wondering if I can select the value of a column if the column exists and just select null otherwise.  In other words I'd like to "lift" the select statement to handle the case when the column doesn't exist.</p>
  
  <pre><code>SELECT uniqueId
      ,  columnTwo
      ,  /*WHEN columnThree exists THEN columnThree ELSE NULL END*/ AS columnThree
  FROM (subQuery) s
  </code></pre>
  
  <p>Note, I'm in the middle to solidifying my data model and design.  I hope to exclude this logic in the coming weeks, but I'd really like to move beyond this problem right because the data model fix is a more time consuming endeavor than I'd like to tackle now.</p>
  
  <p>Also note, I'd like to be able to do this in one query.  So I'm not looking for an answer like </p>
  
  <blockquote>
    <p>check what columns are on your sub query first.  Then modify your
    query to appropriately handle the columns on your sub query.</p>
  </blockquote>