Show the name of customers with a mailshot.
select t2.customer_name from mailshot_customers as t1 join customers as t2 on t1.customer_id = t2.customer_id
Order them by the number of mailshots in descending order.
select t2.customer_name from mailshot_customers as t1 join customers as t2 on t1.customer_id = t2.customer_id group by t1.customer_id order by count(*) desc
Who has the most mailshots among them?
select t2.customer_name from mailshot_customers as t1 join customers as t2 on t1.customer_id = t2.customer_id group by t1.customer_id order by count(*) desc limit 1