title: Oracle SQL combine two columns into one without UNION
body: <p>In ORACLE 11G I have a SELECT with this kind of output:</p>
  
  <pre><code>COL_A       COL_B
  -------     -------
  BCS         (null)
  (null)      FFG
  VKD         FIU
  ...
  </code></pre>
  
  <p>I would like to get this:</p>
  
  <pre><code>MY_ALIAS
  ---------
  BCD
  FFG
  VKD
  FIU
  ...
  </code></pre>
  
  <p>COL_A and COL_B are from the same table. The query is a bit complex.</p>
  
  <p>As you can see there are 3 types of row: COL_A with a value and COL_B null, the oposite and finally both columns with a value.</p>
  
  <p>I know this can be done with a UNION this way:</p>
  
  <pre><code>SELECT MY_ALIAS FROM (
  
      SELECT (COL_A) AS MY_ALIAS
      WHERE &lt;lots of conditions here&gt;
          AND COL_A IS NOT NULL
      UNION
      SELECT (COL_B) AS MY_ALIAS
      WHERE &lt;lots of conditions here&gt;
          AND COL_B IS NOT NULL
  )
  </code></pre>
  
  <p>This way the conditions in WHERE clause would be exactly the same and I would like to avoid it.</p>
  
  <p><strong>Is there any way to achieve that without duplicating  and no PL/SQL, just a SELECT query?</strong></p>
  
  <p><em>Edited to remove the need to avoid the UNION clause. The main purpose of my answer is to avoid the duplication of .</em></p>
  
  <p>Thanks!</p>