title: MySQL behavior of ON DUPLICATE KEY UPDATE for multiple UNIQUE fields
body: <p>From MySQL 4.1.0 onwards, it is possible to add <code>ON DUPLICATE KEY UPDATE</code> statement to specify behavior when values inserted (with <code>INSERT</code> or <code>SET</code> or <code>VALUES</code>) are already in destination table w.r.t. <code>PRIMARY KEY</code> or some <code>UNIQUE</code> field. If value for <code>PRIMARY KEY</code> or some <code>UNIQUE</code> field are already in table, <code>INSERT</code> is replaced by an <code>UPDATE</code>.</p>
  
  <ul>
  <li><p>How does <code>ON DUPLICATE KEY UPDATE</code> behave in case there are multiple
  <code>UNIQUE</code> fields in my table ?</p></li>
  <li><p>Can I have one update only, if either <code>UNIQUE</code> field is matched ?</p></li>
  <li><p>Can I have an update only if both <code>UNIQUE</code> fields are matched simultaneously ?</p></li>
  </ul>