title: Zero-or-one to zero-or-one relationship
body: <p>Is there a way to create a <code>0 or 1</code> to <code>0 or 1</code> relationship in a database?</p>
  
  <p>For example, let's say I have a table of managers and a table of artists. A manager can only manage one artist, and an artist can only have one manager. 
  However, a manager can be without an artist (i.e. looking for an artist to manage) and an artist might not have a manager.</p>
  
  <p>Is there any way to do this? Creating a foreign key in either table would just create a <code>1 to many</code> relationship.</p>