title: SUM multiple columns across multiple tables grouping on date
body: <p>I have a sales and expenses table with rows similar to this</p>
  
  <h2>SALES</h2>
  
  <pre><code>invoiced_on | total | ....
  ---------------------------
  2010-10-01  | 500.00
  2010-10-09  | 700.00
  ...
  </code></pre>
  
  <h2>EXPENSES</h2>
  
  <pre><code>submitted_on | amount
  ---------------------
  2010-10-01   | 100.00
  2010-10-09   | 200.00
  ...
  </code></pre>
  
  <p>Ideally I would select three columns: </p>
  
  <ul>
  <li>week</li>
  <li>sales_total</li>
  <li>expenses_total</li>
  </ul>
  
  <p>I'm not familiar with how to perform group_by on multiple aggregated columns.</p>