title: When is referential integrity not appropriate?
body: <p>I understand the need to have referential integrity for limiting specific values on entry or possibly preventing them from removal upon a request of deletion. However, I am unclear as to a valid use case which would exclude this mechanism from always being used.</p>
  
  <p>I guess this would fall into several sub-questions:</p>
  
  <ol>
  <li>When is referential integrity not appropriate?</li>
  <li>Is it appropriate to have fields containing multiple and/or possibly incomplete subsets of a foreign key's list?</li>
  <li>Typically, should this be a schema structure design decision or an interface design decision? (Or possibly neither or both)</li>
  </ol>
  
  <p>Thoughts?</p>