title: Calculate running total per month
body: <p>Let's say I have this table:</p>
  
  <pre><code>id; date; units
  1; Jan 1; 1
  2; Jan 2; 4
  3; Feb 9; 6
  4; Mar 1; 1
  5; Mar 4; 2
  </code></pre>
  
  <p>How can I now calculate the "accumulated_month" column accordingly? The calculation should start anew with every new month.</p>
  
  <pre><code>id; date; units; accumulated_month
  1; Jan 1; 1; 1
  2; Jan 2; 4; 5
  3; Feb 9; 6; 6
  4; Mar 1; 1; 1
  5; Mar 4; 2; 3
  </code></pre>
  
  <p>All I manage to get is this:</p>
  
  <pre><code>id; date; units; accumulated_month
  1; Jan 1; 1; 5
  2; Jan 2; 4; 5
  3; Feb 9; 6; 6
  4; Mar 1; 1; 3
  5; Mar 4; 2; 3
  </code></pre>