Can you show me the first names of customers and the number of  accounts they have?
select t2.customer_first_name, count ( * ) from accounts as t1 join customers as t2 on t1.customer_id = t2.customer_id group by t1.customer_id
Which customers have at least 2 accounts? List their first names please.
select t2.customer_first_name from accounts as t1 join customers as t2 on t1.customer_id = t2.customer_id group by t1.customer_id having count ( * ) > = 2