问实现可选关系(如果FK不为null，则强制执行)

EN

Person( person_id, name, car_id )
CompanyCar( car_id, make, model, colour, plate )

class Person{
    int person_id,
    string name,
    int? car_id,
    CompanyCar companyCar //object references may hold null
}

SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

SET ANSI_PADDING ON
GO

CREATE TABLE [dbo].[Person](
    [PersonID] [int] NOT NULL,
    [Name] [varchar](100) NOT NULL,
    [CarID] [int] NULL
 CONSTRAINT [PK_Person] PRIMARY KEY CLUSTERED 
(
    [PersonID] ASC
)WITH FILLFACTOR = 92 ON [PRIMARY]
) ON [PRIMARY]

GO

SET ANSI_PADDING OFF
GO

ALTER TABLE [dbo].[Person]  WITH NOCHECK ADD  CONSTRAINT [FK_Person_CompanyCar] FOREIGN KEY([CarID])
REFERENCES [dbo].[CompanyCar] ([CarID])
GO