在外键列中设置空值？

Question

我有这张桌子

CREATE TABLE [dbo].[CityMaster](
    [CityID] [int] NOT NULL,
    [City] [varchar](100) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL,
    [BranchId] [varchar](10) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL CONSTRAINT [DF__CityM__Branc__74444068]  DEFAULT ((0)),
    [ExternalBranchId] [varchar](max) COLLATE SQL_Latin1_General_CP1_CI_AS NULL,
 CONSTRAINT [PK_CityMaster] PRIMARY KEY CLUSTERED 
(
    [City] ASC,
    [BranchId] ASC
),
 CONSTRAINT [uk_citymaster_cityid_branchid] UNIQUE NONCLUSTERED 
(
    [CityID] ASC,
    [BranchId] ASC
)
)

和另一张桌子

CREATE TABLE [dbo].[CustomerMaster](
    [ID] [int] NOT NULL,
    [CustomerCode] [varchar](20) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL,
    [CustomerName] [varchar](50) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL,
    [CustomerAddress] [varchar](100) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL,
    [CustomerPhone] [varchar](max) COLLATE SQL_Latin1_General_CP1_CI_AS NULL,
    [CustomerEmailId] [varchar](100) COLLATE SQL_Latin1_General_CP1_CI_AS NULL,
    [CustomerPriority] [int] NOT NULL CONSTRAINT [DF_CustomerMaster_CustomerPriority]  DEFAULT ((0)),
    [CustomerRegisterDate] [datetime] NULL,
    [CustomerIsActive] [bit] NULL CONSTRAINT [DF_CustomerMaster_CustomerIsActive]  DEFAULT ((1)),
    [BranchId] [varchar](10) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL CONSTRAINT [DF__CustomerM__Branc__67DE6983]  DEFAULT ((0)),
    [CityId] [int] NULL CONSTRAINT [DF_CustomerMaster_CityId]  DEFAULT ((0)),
 CONSTRAINT [PK_CustomerMaster] PRIMARY KEY CLUSTERED 
(
    [CustomerCode] ASC,
    [BranchId] ASC
)
) ON [PRIMARY]

ALTER TABLE [dbo].[CustomerMaster]  WITH CHECK ADD  CONSTRAINT [fk_cdCityId_CityId] FOREIGN KEY([CityId], [BranchId])
REFERENCES [dbo].[CityMaster] ([CityID], [BranchId])

ALTER TABLE [dbo].[CustomerMaster] CHECK CONSTRAINT [fk_cdCityId_CityId]

如您所见,有一个外键CityId, BranchId.我遇到的问题是,如果用户没有进入他的城市(他可以选择不,这个字段不是强制性的,那么这CityId将是空白的,当我尝试在CustomerMaster表中插入此值时,我得到此错误说

INSERT语句与FOREIGN KEY约束"fk_cdCityId_CityId"冲突.冲突发生在数据库"TestDatabase",表"dbo.CityMaster"中.该语句已终止.

所以,我想知道一种绕过这个的方法.我知道如果将唯一或主键列引用为外键,则它不能为空.但是,当我设定的时候 on delete set null呢？在这种情况下,如果该行从删除,CityMaster将它设置为null在CustomerMaster(我的意思是它的所有引用).那么,如果可能的原因以及如何null手动设置此外键中的值？如果这无法通过任何方式实现,那么绕过我描述的情况的最佳方法是什么？

Answer 1

如果CityId设置为NULL,则不会检查外键约束,一切都会好.

在另一方面,如果CityId是0(比如,因为您所指定DEFAULT ((0))就可以了...),并没有匹配的行CityMaster为0,BranchId,那么这将导致确实的约束检查失败.

仅当涉及的所有列都是非列时,才会检查复合键上的外键约束NULL.