title: SQL query: Growth of users per month in percentage
body: <p>I have this easy query that calculates the number of user registrations per month.</p>
  
  <pre class="lang-sql prettyprint-override"><code>SELECT TO_CHAR(created_at, 'YYYY-MM') AS month, COUNT(user_id)
  FROM users
  GROUP BY month
  ORDER BY month DESC 
  </code></pre>
  
  <p>What I would like to have is for every month the growth in percentages compare to the previous month.</p>