title: Subtract value of a field from a count(*) query
body: <p>I have three tables:</p>
  
  <ol>
  <li><code>Plans(PlanID(key), Capacity)</code></li>
  <li><code>CustomerInProject(VahedID, cinpid(key))</code></li>
  <li><code>Vahed(VahedID(key), PlanID,...)</code></li>
  </ol>
  
  <p>This query shows number of houses with the same <code>PlanID</code> (map)  that people hired:</p>
  
  <pre><code>select 
      count(*) 
  from
      Vahed as v2,
      CustomerInProject 
  where 
      CustomerInProject.VahedID = v2.VahedID
  group by PlanID 
  </code></pre>
  
  <p><code>Plans</code> has an <code>int</code> field named <code>Capacity</code>. I want to subtract <code>Capacity</code> from the above query. How can I do that?</p>