CREATE TABLE Discount_Coupons (coupon_amount VARCHAR, coupon_id VARCHAR)
CREATE TABLE customers (coupon_id VARCHAR, good_or_bad_customer VARCHAR)

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'
SELECT "T1"."coupon_amount" FROM "Discount_Coupons" AS "T1" JOIN "customers" AS "T2" ON "T1"."coupon_id" = "T2"."coupon_id" AND "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" AND "T2"."good_or_bad_customer" = 'bad'