title: Any example of a necessary nullable foreign key?
body: <pre><code>Customers
   customer_id
  
  Orders
   order_id
   customer_id fk
  </code></pre>
  
  <p>If I have two tables and define a foreign key on customer_id in the Orders table, by allowing it to be null I am saying that I can have an order that does not have a customer associated with it. As such, the notion of a nullable foreign key seems at odds with the purpose of a foreign key, which is to enforce this constraint.</p>
  
  <p>Is there a simple example of a situation in which a nullable foreign key would be necessary? Or an argument in favor of permitting them?</p>