SELECT DISTINCT
  p.id AS Identyfikator,
  p.first_name AS Imię,
  p.last_name  AS Nazwisko,
  c.name       AS [Nazwa nieopłaconej konferencji]
FROM dbo.Participant AS p
  LEFT JOIN  dbo.BookingDayReservationParticipant AS b ON p.id = b.participant_id
  INNER JOIN  dbo.BookingDay AS bd ON bd.conference_day_id = b.id
  INNER JOIN  dbo.ConferenceDay AS cd ON cd.id = bd.conference_day_id
  INNER JOIN  dbo.Conference AS c ON c.id = cd.conference_id
WHERE (b.paid_date IS NULL)