title: Hibernate count rows with some criterias
body: <p>Suppose I have a table Person and i want to count all those people whose "birthDate" is not null and they are a student.
  Assuming i have two columns :</p>
  
  <pre><code>birthDate Date (can be null)
  isStudent boolean (default: false)
  </code></pre>
  
  <p>How can i do this using hibernate.. ?</p>