How many customers have accounts?
select count ( * ) from accounts as t1 join customers as t2 on t1.customer_id = t2.customer_id
What is the customer first, last name and id with the most number of accounts?
select t2.customer_first_name, t2.customer_last_name, t1.customer_id from accounts as t1 join customers as t2 on t1.customer_id = t2.customer_id group by t1.customer_id order by count ( * ) desc limit 1
What is the customer first, last name and id with the least number of accounts?
select t2.customer_first_name, t2.customer_last_name, t1.customer_id from accounts as t1 join customers as t2 on t1.customer_id = t2.customer_id group by t1.customer_id order by count ( * ) asc limit 1