title: SQL give me the courses where users inside are less than 30
body: <p>I'm looking for a solution to get a list of courses that have less than <code>30</code> users inside.
  I've looked everywhere, but I can not get through it. I'm not very practical with mySQL.</p>
  
  <p>I have 3 tables:</p>
  
  <pre><code>1) TABLE course
  _ID, COURSECODE
  
  2) TABLE course_user:
  _ID, ID_COURSE, ID_USER
  
  3) TABLE user:
  _ID
  </code></pre>
  
  <p>How can I get the <code>COURSECODE</code> only of courses that have less than <code>30</code> users?</p>
  
  <p>Thank you very much ... sorry for my bad english.</p>