title: SQL Query: number per quarter hour
body: <p>I have a sql table with 4 columns (id (char), ref(varchar), received(timestamp), sent(timestamp)).
  I would like to make a sql query to get the number of references per quarter of an hour according to the 'received' column.
  I've been searching some time but haven't find a real solution to do it in a query.
  Is there someone who has any idea how to do this?
  Thanks in advance.</p>
  
  <p>Kind Regards,</p>
  
  <p>Tom</p>