title: Is it fine to have foreign key as primary key?
body: <p>I have two tables:</p>
  
  <ul>
  <li>User (username, password)</li>
  <li>Profile (profileId, gender, dateofbirth, ...)</li>
  </ul>
  
  <p>Currently I'm using this approach: each Profile record has a field named "userId" as <em>foreign key</em> which links to the User table. When a user registers, his Profile record is automatically created.</p>
  
  <p>I'm confused with my friend suggestion: to have the "userId" field as the <em>foreign</em> and <em>primary</em> key and delete the "profileId" field. Which approach is better?</p>