Count the number of participants for each event.
select event_id,count(*) from participants_in_events group by event_id
Among those, which event id has the least participants?
select event_id from participants_in_events group by event_id order by count(*) asc limit 1
What service id is it?
select service_id from events where event_id = (select event_id from participants_in_events group by event_id order by count(*) asc limit 1)
Also list the service type of it.
select t3.service_id, t4.service_type_code from participants as t1 join participants_in_events as t2 on t1.participant_id = t2.participant_id join events as t3 on t2.event_id = t3.event_id join services as t4 on t3.service_id = t4.service_id group by t3.service_id order by count(*) asc limit 1