允许外键为空

Question

我正在尝试在两个表之间创建关系，其中表 A 中的一列是表 B 中一列的外键。但是，表 B 中有一行当前在该列中为空，这给了我一个错误. 然而，从谷歌搜索这个问题来看，普遍的共识似乎是 null 是外键字段中可接受的值。但是由于错误消息，我仍然无法在 SSMS 中创建这种关系。我怎样才能绕过这个？

错误：

Unable to create relationship 'FK_RELATIONSHIP'. 
The ALTER TABLE statement conflicted with the FOREIGN KEY 
constraint "FK_RELATIONSHIP". The conflict occurred in database "DATABASE", 
table "TABLE A", column 'COLUMN 1'.

请求的 SQL 代码：

USE [DATABASE]
GO
/****** Object:  Table [dbo].[TABLEA]    Script Date: 10/02/2012 17:44:42 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[TABLEA](
[COLUMN1] [nvarchar](4) NOT NULL,
CONSTRAINT [PK_TABLEA] PRIMARY KEY CLUSTERED 
(
[COLUMN1] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF,        ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object:  Table [dbo].[TABLEB]    Script Date: 10/02/2012 17:44:42 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[TABLEB](
[COLUMN1] [nvarchar](15) NOT NULL,
[COLUMN2] [nvarchar](4) NULL,
 CONSTRAINT [PK_TABLEB] PRIMARY KEY CLUSTERED 
(
[COLUMN1] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF,   ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
GO

谢谢

Answer 1

对于任何在这里遇到同样问题的人。空单元格与 NULL 单元格不同。空单元格是 '' 不是 NULL！