title: How to handle optional fields in MySQL?
body: <p>I have a MySQL table that records classified listings. We don't force users to join to post a listing, and therefore the listing will not always have a <code>user_id</code> associated with it.</p>
  
  <p>I therefore need a method of recording the poster's email if they are not signed in.</p>
  
  <p>Is it bad practice to create a column <code>email</code> that will sometimes be blank and sometimes be filled?</p>
  
  <p>Or is there a better way to go about this that I don't realize?</p>