Show the customer id numbers for those who have events.
select t1.customer_id from customers as t1 join customer_events as t2 on t1.customer_id = t2.customer_id
How about those with at least 3?
select t1.customer_id from customers as t1 join customer_events as t2 on t1.customer_id = t2.customer_id group by t1.customer_id having count(*) >= 3
Also show customer details.
select t1.customer_id, t1.customer_details from customers as t1 join customer_events as t2 on t1.customer_id = t2.customer_id group by t1.customer_id having count(*) >= 3