title: Can peewee nest SELECT queries such that the outer query selects on an aggregate of the inner query?
body: <p>I'm using peewee2.1 with python3.3 and an sqlite3.7 database.</p>
  
  <p>I want to perform certain SELECT queries in which:</p>
  
  <ol>
  <li>I first select some aggregate (count, sum), grouping by some id column; then </li>
  <li>I then select from the results of (1), aggregating over its aggregate. Specifically, I want to count the number of rows in (1) that have each aggregated value.</li>
  </ol>
  
  <p>My database has an 'Event' table with 1 record per event, and a 'Ticket' table with 1..N tickets per event. Each ticket record contains the event's id as a foreign key. Each ticket also contains a 'seats' column that specifies the number of seats purchased. (A "ticket" is really best thought of as a purchase transaction for 1 or more seats at the event.)</p>
  
  <p>Below are two examples of working SQLite queries of this sort that give me the desired results:</p>
  
  <pre><code>SELECT ev_tix, count(1) AS ev_tix_n FROM 
  (SELECT count(1) AS ev_tix FROM ticket GROUP BY event_id)
  GROUP BY ev_tix
  
  SELECT seat_tot, count(1) AS seat_tot_n FROM
  (SELECT sum(seats) AS seat_tot FROM ticket GROUP BY event_id)
  GROUP BY seat_tot
  </code></pre>
  
  <p>But using Peewee, I don't know how to select on the inner query's aggregate (count or sum) when specifying the outer query. I can of course specify an alias for that aggregate, but it seems I can't use that alias in the outer query.</p>
  
  <p>I know that Peewee has a mechanism for executing "raw" SQL queries, and I've used that workaround successfully. But I'd like to understand if / how these queries can be done using Peewee directly.</p>