title: Show html dropdown list according chosen options
body: <p>I'm building an small web app querying a database with python CGI. The logic is:
  HTML Form - Python CGI - SQL query - python cgi prints results.  </p>
  
  <p>At the moment it works, but now I'm stuck creating the forms. What I want to do is auto fill the second form according the results of the first one. Example:  </p>
  
  <p>First Dropdown one options: Car, Motorcycle, BUS, Train.<br>
  Second dropdown: If I chose car, sow only car options. If train, only train options...  </p>
  
  <p>I know how to make it with <code>SQL ('WHERE type = 'car'')</code> But I don't know how to fill the html form.</p>
  
  <p>I'm not using any framework, just HTML, Python (instead of php) and Postgresql</p>