CREATE TABLE dbo.Payment(
    PaymentId     BIGINT IDENTITY(1,1) NOT NULL,
    ReservationId BIGINT NULL,
    Amount        DECIMAL(12,2) NOT NULL,
    PaidAt        DATETIME2(0) NULL,
    CONSTRAINT PK_Payment PRIMARY KEY (PaymentId),
    CONSTRAINT FK_Payment_Reservation FOREIGN KEY (ReservationId)
        REFERENCES dbo.Reservation(ReservationId)
);

CREATE TABLE dbo.Payment(
    PaymentId     BIGINT IDENTITY(1,1) NOT NULL,
    ReservationId BIGINT NOT NULL,
    Amount        DECIMAL(12,2) NOT NULL,
    PaidAt        DATETIME2(0) NULL,
    CONSTRAINT PK_Payment PRIMARY KEY (PaymentId),
    CONSTRAINT FK_Payment_Reservation FOREIGN KEY (ReservationId)
        REFERENCES dbo.Reservation(ReservationId)
);

CREATE TABLE dbo.Payment(
    PaymentId     BIGINT IDENTITY(1,1) NOT NULL,
    ReservationId BIGINT NOT NULL,
    Amount        DECIMAL(12,2) NOT NULL,
    PaidAt        DATETIME2(0) NULL,
    CONSTRAINT PK_Payment PRIMARY KEY (PaymentId),
    CONSTRAINT UQ_Payment_Reservation UNIQUE (ReservationId),
    CONSTRAINT FK_Payment_Reservation FOREIGN KEY (ReservationId)
        REFERENCES dbo.Reservation(ReservationId)
);

ALTER TABLE dbo.Reservation
ADD PaymentId BIGINT NULL,
    CONSTRAINT FK_Reservation_Payment FOREIGN KEY (PaymentId)
        REFERENCES dbo.Payment(PaymentId),
    CONSTRAINT UQ_Reservation_Payment UNIQUE (PaymentId);