title: Difference between sum of two record sets grouped by the same column
body: <p>I need to write a SQL query to calculate the difference in revenues between Europe and Asia. I am constrained by the app dev and cannot calculate this in 2 steps (compute the totals in each continent separately then calculate the difference)</p>
  
  <p>Here is the db:</p>
  
  <pre><code>Sales(s_key, s_key, s_product, s_date, s_amount, s_quantity) 
  
  Customer(c_key, c_name, c_city, c_country, c_continent)
  </code></pre>
  
  <p>Can you help me extend this query which calculates the revenue in Europe to get difference with revenue in Asia?
  Thank you!</p>
  
  <pre><code>Select Sum(s_amount)
  From Sales
  Where c_continent = 'Europe'
  Group By c_continent
  </code></pre>