How many participants are there for event 3?
select count(*) from events as t1 join participants_in_events as t2 on t1.event_id = t2.event_id where t1.event_id = 3
How many events have 3 participants, list the event id and details.
select t1.event_id, t1.event_details from events as t1 join participants_in_events as t2 on t1.event_id = t2.event_id group by t1.event_id having count(*) = 3
What about more than one participants?
select t1.event_id, t1.event_details from events as t1 join participants_in_events as t2 on t1.event_id = t2.event_id group by t1.event_id having count(*) > 1