title: Can table columns with a Foreign Key be NULL?
body: <p>I have a table which has several ID columns to other tables.</p>
  
  <p>I want a foreign key to force integrity <strong>only</strong> if I put data in there. If I do an update at a later time to populate that column, then it should also check the constraint.</p>
  
  <p>(This is likely database server dependant, I'm using MySQL &amp; InnoDB table type)</p>
  
  <p>I believe this is a reasonable expectation, but correct me if I am wrong.</p>