title: MySQL : Find out what the state was of a row on a certain date
body: <p>We have a database which contains</p>
  <ul>
  <li>calls and the state of a call (incoming / Answered / Callback / etc)</li>
  <li>History of state transition : ex. on 1st of March  at 9 AM call-id 100 went from state incoming to state answered</li>
  </ul>
  <p>I would like to write a query which could summarize how many calls were on Callback at a given date.
  Although I have quite some experience in MySQL, I lack the knowledge to find the right function and Google didn't really help me to find a solution</p>