title: Is string or int preferred for foreign keys?
body: <p>I have a user table with <code>userid</code> and <code>username</code> columns, and both are unique.</p>
  
  <p>Between <code>userid</code> and <code>username</code>, which would be better to use as a foreign key and why?<br />
  My Boss wants to use string, is that ok?</p>