custom_left_join.php

    $result = $this
      ->createQuery('r')
      ->select('r.*, e.*')
      ->from('BookingResource r')
      ->leftJoin(
        'r.Entries e ON '.
        '( ( e.ending_at   > :start AND e.ending_at    <= :end   ) OR '.
        '  ( e.starting_at < :end   AND e.starting_at  >= :start ) OR '.
        '  ( e.starting_at < :start AND e.ending_at    >  :end   ) )'
      )
      ->orderBy('e.starting_at')
      ->where('r.category_id = :category_id')
      ->execute(
        array(
          ':start' => $start->get(),
          ':end'   => $end->get(),
          ':category_id' => $category['id'],
        )
      );