title: XML element unique id Schema repersentation
body: <p>My XML looks likes this :</p>
  
  <pre><code>&lt;company&gt;
    &lt;employee id="1"&gt;Larsen&lt;/employee&gt;
    &lt;employee id="2"&gt;Smith&lt;/employee&gt;
    &lt;employee id="3"&gt;Sam&lt;/employee&gt;
  &lt;/company&gt;
  </code></pre>
  
  <p>How to write a xml schema so that employee element is defined such a way that each employee has a unique id attribute (no two employee elements can have same value for id attribute)</p>