This is the Monster Market database. Open the customers tab: email is unique.

1. Add customer 13: Ken Ito, from JP, signed up on 2025-06-10, with the email ava.chen@example.com. The database refuses it. Which constraint does the error name?
2. Customer 10 has no email. Before you run it: will the database accept customer 13 with no email? Answer yes or no.
3. Add customer 13 with no email.
4. Create a table coupons: id (whole number, the primary key), code (text, required, and never the same for two coupons) and percent_off (whole number).
5. Each customer may use each coupon only once. Create a table coupon_uses with customer_id, coupon_id (whole numbers, required) and used_on (date, required), where the database refuses a second use of the same coupon by the same customer.