title: Query with equation
body: <p>I have 3 queries that return 3 values. I'd Like to join the queries to perform the following expression:</p>
  
  <pre><code>(MgO + CaO)/SiO2
  </code></pre>
  
  <p>How can I do that?</p>
  
  <p>MgO:</p>
  
  <pre><code>SELECT sampled_date, result 
  FROM AF_VW WHERE element = 'MgO' AND ROWNUM = 1 ORDER BY sampled_date DESC;
  </code></pre>
  
  <p>CaO:</p>
  
  <pre><code>SELECT sampled_date, result 
  FROM AF_VW WHERE element = 'CaO' AND ROWNUM = 1 ORDER BY sampled_date DESC;
  </code></pre>
  
  <p>SiO2:</p>
  
  <pre><code>SELECT sampled_date, result 
  FROM AF_VW WHERE element = 'SiO2' AND ROWNUM = 1 ORDER BY sampled_date DESC;
  </code></pre>