title: How to do a division from calculated values from one table in a single query
body: <p>I have the following SQL query and I want to calculate a derivated field, A/B:</p>
  
  <pre><code>select center, month, sum(countA) as A, sum(countB) as B, A/B 
  from counter
  group by center, month
  </code></pre>
  
  <p>However, this sentence throws an error, and I would wish to not repeat the sum again. Is there any way to do that on a single query?</p>