title: SQL Query cannot figure out issue
body: <p>The question I'm trying to answer is </p>
  
  <blockquote>
    <p>Write a SELECT statement that returns the CategoryName column from the
    Categories table. Return one row for each category that has never been
    assigned to any product in the Products table. To do that, use a
    subquery introduced with the NOT EXISTS operator.</p>
  </blockquote>
  
  <p>The Categories table includes the columns: <code>CategoryID</code> and <code>CategoryName</code></p>
  
  <p>The Products table includes the columns: <code>ProductID</code>,<code>CategoryID</code>,<code>ProductCode</code>,<code>ProductName</code>,<code>Description</code>,<code>ListPrice</code>,<code>DiscountPercent</code>,<code>DateAdded</code></p>
  
  <p>This is what I've tried: </p>
  
  <pre><code>SELECT CategoryName
  From Categories
  Where NOT EXISTS(SELECT CategoryID FROM Products WHERE CategoryID IS NOT NULL)
  </code></pre>
  
  <p>Anything helps, thanks!</p>