title: How can I include a non existing value in GROUP BY with COUNT(*)?
body: <p>Considering this simple table called <code>events</code></p>
  
  <pre><code>date       | reserved
  -----------+---------
  2019-11-13 | 1
  2019-11-14 | 0
  2019-11-15 | 1
  </code></pre>
  
  <p>Running the following query, I can retrieve the count of not reserved and reserved events:</p>
  
  <pre><code>SELECT count(*) FROM `events` GROUP BY `reserved`
  </code></pre>
  
  <pre class="lang-none prettyprint-override"><code>1
  2
  </code></pre>
  
  <p>However, if all events are reserved I will get a single row with a single column:</p>
  
  <pre class="lang-none prettyprint-override"><code>3
  </code></pre>
  
  <p>What is the simplest way to get instead:</p>
  
  <pre class="lang-none prettyprint-override"><code>0
  3
  </code></pre>