CREATE TABLE participants (participant_id DECIMAL, participant_type_code TEXT, participant_details TEXT)
CREATE TABLE services (service_id DECIMAL, service_type_code TEXT)
CREATE TABLE participants_in_events (event_id DECIMAL, participant_id DECIMAL)
CREATE TABLE events (event_id DECIMAL, service_id DECIMAL, event_details TEXT)

How many events did not have any participants?
SELECT COUNT(*) FROM events WHERE NOT event_id IN (SELECT event_id FROM participants_in_events)
SELECT COUNT(*) FROM "events" WHERE NOT "event_id" IN (SELECT "event_id" FROM "participants_in_events")