Show the names of customers.
select customer_name from customers
How about the names for those who have an outcome code 'Order' of mailshots.
select t2.customer_name from mailshot_customers as t1 join customers as t2 on t1.customer_id = t2.customer_id where outcome_code = 'Order'
Only show the names of those who have at least two such codes.
select t2.customer_name from mailshot_customers as t1 join customers as t2 on t1.customer_id = t2.customer_id where outcome_code = 'Order' group by t1.customer_id having count(*) >= 2