Slechts één actief voor elke relatiebeperking

Soort

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)
);

Zie fiddle https://dbfiddle.uk/?rdbms=sqlserver_2017&fiddle=596b30905d02a9e5c799b16da5fff5ab">https://dbfiddle.uk/?rdbms=sqlserver_2017&fiddle=