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)
CREATE TABLE participants (participant_id DECIMAL, participant_type_code TEXT, participant_details TEXT)

What are all the distinct participant ids who attended any events?
SELECT COUNT(DISTINCT participant_id) FROM participants_in_events
SELECT COUNT(DISTINCT "participant_id") FROM "participants_in_events"