title: How to do UNION in flexible query
body: <p>I want to merge <code>catetegory_id</code> and <code>subCategory_id</code> into one column. But I don't know how to do it in flexible query. Below code is what I want to do </p>
  
  <pre><code>SELECT * FROM 
  
  (SELECT
      ccr.targetpk as pk
      FROM
      Categories cc
      JOIN Cat2CatRel ccr ON cc.pk = ccr.sourcepk
      WHERE cc.p_code = '0106') 
  
  UNION  SELECT CATE.pk as pk FROM CATEGORIES cate WHERE CATE.p_code = '0106'
  </code></pre>
  
  <p>I need a flexible query which works same as above code.</p>