Only one active for each relationship constraint - sql

Question

So i have this problem with my assignment.
I have two entities: Order, Gifr_cupon.
I have two tables: Orders, Gift_Cupons.
Each order can have many cupons or none. Each Cupon is bound to no or an single order. But only one cupon can be active for each order.
How to enforce this by constraint?
Heres a logical and ER view with DDL:
DLL:
CREATE TABLE gift_cupons (
cupon_id INTEGER NOT NULL,
order_order_id INTEGER,
active INTEGER NOT NULL
);
ALTER TABLE gift_cupons ADD CONSTRAINT gift_cupon_pk PRIMARY KEY ( cupon_id
);
ALTER TABLE gift_cupons ADD CHECK gift_cupon_check CHECK(active IS NULL OR ( active >= 0 AND active <=1 ) );
CREATE TABLE orders (
order_id INTEGER NOT NULL
);
ALTER TABLE orders ADD CONSTRAINT order_pk PRIMARY KEY ( order_id );
ALTER TABLE gift_cupons
ADD CONSTRAINT gift_cupon_order_fk FOREIGN KEY ( order_order_id )
REFERENCES orders ( order_id );

Answer 1

Kind of
Cupon - is bound to -> Order;
Order - has active -> Cupon;
Cupon (
Id PK,
orderId FK Order.Id,
Unique ( Id, orderId) -- any superset of PK is unique
);
Order (
Id PK
ActiveCuponId,
(Id, ActiveCuponId) FK Cupon( OrderId, Id)
);
See fiddle https://dbfiddle.uk/?rdbms=sqlserver_2017&fiddle=596b30905d02a9e5c799b16da5fff5ab

Answer 2

Remove the ACTIVE column from the gift_cupons table, and replace this state by a foreign key in the orders table, as in:
CREATE TABLE gift_cupons (
cupon_id INTEGER NOT NULL,
order_order_id INTEGER,
);
ALTER TABLE gift_cupons ADD CONSTRAINT gift_cupon_pk PRIMARY KEY ( cupon_id
);
CREATE TABLE orders (
order_id INTEGER NOT NULL
active_cupon INTEGER -- nullable
);
ALTER TABLE orders ADD CONSTRAINT order_pk PRIMARY KEY ( order_id );
ALTER TABLE gift_cupons
ADD CONSTRAINT gift_cupon_order_fk FOREIGN KEY ( order_order_id )
REFERENCES orders ( order_id );
alter table orders
add constraint order_active_cupon_fk foreign key (active_cupon)
references gift_cupons (cupon_id);