title: INSERT statement conflicted with COLUMN FOREIGN KEY
body: <p>I have a FK in my table that looks like this:</p>
  
  <p><img src="https://i.stack.imgur.com/f6NGX.png" alt="FK"></p>
  
  <p>Clearly, I should be able to insert <code>NULL</code> into this column as well as any value that exists in the parent table.</p>
  
  <p>But when I try to insert <code>NULL</code>, I get the following error:</p>
  
  <blockquote>
    <p>The INSERT statement conflicted with the FOREIGN KEY constraint
    "users_fk". The conflict occurred in database "mydatabase", table
    "dbo.country", column 'country_id'.</p>
  </blockquote>