title: How do I use a subquery to show a column that has never been assigned to a different table?
body: <p>I'm trying to Write a SELECT statement that returns the category_name column from the Categories table and returns one row for each category that has never been assigned to a product in the Products table. there are four categories in the categories table and
  only three have been used in the products table. How do i make it show the one that
  hasn't been used?
  this is the code I've been trying, but it returns an empty result set.       </p>
  
  <pre><code>select category_name
  from categories c natural join products p
  where not exists (
  select category_name, category_id 
  from categories c natural join products p);
  </code></pre>