SET @STARTDATE = '{STARTDATE}';
SET @ENDDATE = '{ENDDATE}';

SELECT
 CONCAT(se.STARTDATE, '+t') AS 'DATE',
 se.TITLE AS CALENDAR_EVENT,
 CONCAT( c.LASTNAME, ', ', c.FIRSTNAME ) AS CUSTOMER
FROM
 schedule_participants sp
 LEFT JOIN schedule_bookings sb ON sp.BOOKING_ID = sb.BOOKING_ID
 LEFT JOIN schedule_events se ON sb.COURSE_GUID = se.COURSE_GUID
 LEFT JOIN customers c ON sp.CUSTOMER_ID = c.CUSTOMER_ID
WHERE
 se.STARTDATE BETWEEN @STARTDATE AND @ENDDATE
 AND sb.CANCELLED = 0
 AND CONCAT(
  sb.COURSE_GUID,
  sp.CUSTOMER_ID,
  DATE( se.STARTDATE )) NOT IN (
 SELECT
  CONCAT(
      cI.EVENT_SESSION_GUID,
      ci.CUSTOMER_ID,
  DATE( ci.POSTDATE ))
 FROM
  checkins ci
 WHERE
  ci.POSTDATE BETWEEN @STARTDATE AND @ENDDATE
 )
ORDER BY
 se.STARTDATE;