Show the first name and id for all customers with an accout.
select t2.customer_first_name, t1.customer_id from accounts as t1 join customers as t2 on t1.customer_id = t2.customer_id
For each of them, also count the number of accounts.
select t2.customer_first_name, t1.customer_id, count(*) from accounts as t1 join customers as t2 on t1.customer_id = t2.customer_id group by t1.customer_id
Show the first name and id for those with at least 2 accounts.
select t2.customer_first_name, t1.customer_id from accounts as t1 join customers as t2 on t1.customer_id = t2.customer_id group by t1.customer_id having count(*) >= 2