Show me the first name of good customers.
select first_name from customers where good_or_bad_customer = 'good'
What coupon amounts did they own?
select t1.coupon_amount from discount_coupons as t1 join customers as t2 on t1.coupon_id = t2.coupon_id where t2.good_or_bad_customer = 'good'
What about those owned by bad customers?
select t1.coupon_amount from discount_coupons as t1 join customers as t2 on t1.coupon_id = t2.coupon_id where t2.good_or_bad_customer = 'bad'
Show me the coupon amounts owned by both of them.
select t1.coupon_amount from discount_coupons as t1 join customers as t2 on t1.coupon_id = t2.coupon_id where t2.good_or_bad_customer = 'good' intersect select t1.coupon_amount from discount_coupons as t1 join customers as t2 on t1.coupon_id = t2.coupon_id where t2.good_or_bad_customer = 'bad'