CREATE TABLE dbo.Orders
(
     order_id int NOT NULL IDENTITY(1,1) CONSTRAINT PK_ORDERS PRIMARY KEY CLUSTERED,
     order_description varchar(250) NOT NULL
);

CREATE TABLE dbo.Invoices
(
     invoice int NOT NULL CONSTRAINT PK_INVOICES PRIMARY KEY CLUSTERED,
     sales_amount decimal(15,4) NOT NULL CONSTRAINT DF_INVOICES_salesamount DEFAULT (0.00),
     order_id int NULL CONSTRAINT FK_INVOICES_ORDERS FOREIGN KEY REFERENCES dbo.Orders(order_id) ON DELETE SET NULL,
     deleted bit NOT NULL CONSTRAINT DF_INVOICES_deleted DEFAULT (1)
);

select * from sys.foreign_keys