What are the coupon amount of the coupons owned by both good and 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 = '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'