How many events are there?
select count(*) from events
List all the event ids.
select event_id from participants_in_events
Show all the event information if the event has someone participated.
select * from events where event_id in (select event_id from participants_in_events)
How about nobody has participated?
select * from events where event_id not in (select event_id from participants_in_events)
How many are there?
select count(*) from events where event_id not in (select event_id from participants_in_events)