title: avg() of count of selected rows per month in a year
body: <p>I need to return avg() of count of selected rows per month in a year per system from an Access query that looks like that:</p>
  
  <pre><code>SELECT backlog.System, 
      DatePart('yyyy',[Create Date]) AS [Year], 
      DatePart('m',[Create Date]) AS [Month], 
      Count(backlog.[Ticket ID]) AS [Count of Registered] 
  FROM backlog 
  GROUP BY backlog.System, 
      DatePart('yyyy',[Create Date]), 
      DatePart('m',[Create Date]);
  </code></pre>
  
  <p>Results: </p>
  
  <pre><code>- Col1 | Year | Month | Count 
  - System1 | 2013 | 1 | 25 
  - System1 | 2014 | 1 | 12 
  - System1 | 2014 | 2 | 6 
  - System2 | 2013 | 1 | 4 
  - System2 | 2013 | 2 | 56
  </code></pre>
  
  <p>I would like to get this:</p>
  
  <pre><code>- Col1 | Year | Month | Count | Average
  - System1 | 2013 | 1 | 25 | 25
  - System1 | 2014 | 1 | 12 | 9
  - System1 | 2014 | 2 | 6 | 9
  - System2 | 2013 | 1 | 4 | 30
  - System2 | 2013 | 2 | 56 | 30
  </code></pre>