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)
CREATE TABLE services (service_id DECIMAL, service_type_code TEXT)

List the type of the services in alphabetical order.
SELECT service_type_code FROM services ORDER BY service_type_code
SELECT "service_type_code" FROM "services" ORDER BY "service_type_code" NULLS FIRST