title: Using references in MYSQL
body: <p>I got an answer to another question here:</p>
  
  <p><a href="https://stackoverflow.com/questions/3094495/db-schema-for-chats/3094915#3094915">DB Schema For Chats?</a></p>
  
  <p>It's a great answer, but I am not understanding the bit about references. I can do SQL statements but I have never used references.</p>
  
  <ol>
  <li>What are they used for?</li>
  <li>How are they used?</li>
  <li>Give an example please</li>
  </ol>