title: Data type Mismatch in criterion expression
body: <p>I'm entirely new to SQL and keep getting an data type mismatch in criterion expression. Trying to figure out where to make the changes.</p>
  <pre><code>SELECT AVG(Price)
  FROM Book, OrderLine, [Order]
  WHERE OrderDeliveryDate &gt; '2016/05/05' and Order.OrderNo = OrderLine.OrderNo and Book.ISBN = OrderLine.ISBN and PercentageDiscount = 'NULL';
  </code></pre>