title: How to model or query for something that does not exist in relational algebra
body: <p>I want to query the id of all apartments that were never rented.</p>
  <p>I tried something like this:</p>
  <pre><code>    (π a_id (apartments))
  -
      (π a_id
      σ from_date Exists ∧ end_date Exists 
      (rental) ⨝ rental.a_id on apartment.a_id (apartment))
  </code></pre>
  <p>But I think I cannot use Exists or null in relational algebra.</p>
  <p>How could I do it?</p>
  <p>I attach the schema <a href="https://i.stack.imgur.com/cGprt.png" rel="nofollow noreferrer">here</a></p>