title: Group by decade with counts
body: <p>I have a table of <code>Album</code>'s that has a <code>date</code> column named <code>release_date</code>.</p>
  
  <p>I want to get a list of all the decades along with the number of albums released in that decade.</p>
  
  <p>So, the output might be something like:</p>
  
  <ul>
  <li>2010 - 11</li>
  <li>2000 - 4</li>
  <li>1990 - 19</li>
  <li>1940 - 2</li>
  </ul>
  
  <p>Ruby 2.3.1 w/ Rails 5 on Postgres 9.6, FWIW.</p>
  
  <p>This is essentially a followup question to a previous one I had: <a href="https://stackoverflow.com/questions/40538996/get-list-of-month-year-combinations-present-along-with-count">Group by month+year with counts</a></p>
  
  <p>Which may help with the solution...I'm just not sure how to do the grouping by decade.</p>