sql-server – 为什么我的检查约束不能停止这个空插入？

据我所知,支票约束应该只允许：

>代码为空,系统为空.
>代码不为空,系统为1.

我的第一个想法是ANSI NULLS,但是将它们设置为关闭也没有任何区别.

这是我们在应用程序中发现的更大问题的简化示例(系统已针对数字列表(IN,1,2等)进行了检查).我们用一个外键(而不是IN)替换了这个检查和一个新的检查约束,允许null或null都为null;这样做阻止了第三个插入.

IF  EXISTS (SELECT * FROM sys.check_constraints WHERE object_id = OBJECT_ID(N'[dbo].[CK_TestCheck]') AND parent_object_id = OBJECT_ID(N'[dbo].[TestCheck]'))
    ALTER TABLE [dbo].[TestCheck] DROP CONSTRAINT [CK_TestCheck]
GO

IF  EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[TestCheck]') AND type in (N'U'))
    DROP TABLE [dbo].[TestCheck]
GO

SET ANSI_NULLS ON
GO

CREATE TABLE TestCheck(
    [Id] [int] IDENTITY(1,1) NOT NULL,[Code] [varchar](50) NULL,[System] [tinyint] NULL,PRIMARY KEY CLUSTERED ([Id] ASC))
GO

ALTER TABLE [dbo].[TestCheck] WITH CHECK ADD CONSTRAINT [CK_TestCheck] CHECK
(
    ([Code] IS NULL AND [System] IS NULL)   --Both null
    OR
    ([Code] IS NOT NULL AND [System] = 1)   --Both not null ????
)
GO

ALTER TABLE [dbo].[TestCheck] CHECK CONSTRAINT [CK_TestCheck]
GO

--Good Data
insert TestCheck (Code,[System]) Values(null,null);
insert TestCheck (Code,[System]) Values('123',1);

--Query Data
insert TestCheck (Code,null);

--Bad data stopped
insert TestCheck (Code,1);
insert TestCheck (Code,4);

select * from TestCheck
Where
    case when
    (
        ([Code] IS NULL AND [System] IS NULL)           --Both null
        OR
        ([Code] IS NOT NULL AND [System] in (1,2,3))  --Both not null ????
    )
    then 0 else 1 end
     = 1

解决方法

>([Code] IS NULL AND [System] IS NULL)评估为False
>([Code] IS NOT NULL AND [System] IN(1,3))求值为“未定义”

结果未定义

Check Constraint

CHECK constraints reject values that evaluate to FALSE. Because null
values evaluate to UNKNOWN,their presence in expressions may override
a constraint.

您应该将[系统] IN(1,3)的检查更改为ISNULL([系统],0)IN(1,3).

你的检查约束就变成了

ALTER TABLE [dbo].[TestCheck] WITH CHECK ADD CONSTRAINT [CK_TestCheck] CHECK
(
    ([Code] IS NULL AND [System] IS NULL)   --Both null
    OR
    ([Code] IS NOT NULL AND ISNULL([System],0) IN (1,3))   --Both not null ????
)

（编辑：李大同）

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