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

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"