title: relational algebra expression for "is null"
body: <p>What is the relational algebra expression of the query below? I couldn't find the expression for "Is Null".</p>
  
  <pre><code>SELECT reader.name
  FROM reader LEFT JOIN book_borrow ON reader.cardid = book_borrow.cardid
  WHERE book_borrow.cardid Is Null;
  </code></pre>