title: Group by 2 columns and count
body: <p>I have the following MySql table - user_actions:
  id - int
  user_id - int
  timestamp - datetime
  action - int</p>
  
  <p>a data example:</p>
  
  <pre><code>1|5|01.01.2011 21:00:00|1
  2|5|01.01.2011 21:00:00|3
  3|6|01.01.2011 21:00:00|5
  3|6|01.02.2011 21:00:00|5
  </code></pre>
  
  <p>I want to count the number of users who made an action in each day (no matter how many actions), in the above example the output should be:</p>
  
  <pre><code>01.01.2011|2 
  01.02.2011|1 
  </code></pre>
  
  <ul>
  <li>meaning 2 users made actions in 01.01.2011 and one user in 01.02.2011</li>
  </ul>
  
  <p>any thoughts? </p>