Question

Can anyone explain why the third insert (labeled Query Data) in the below code is allowed by SQL Server?

As far as I can tell, the check constraint should only allow:

• Code is null and System is null.
• Code is not null and System is 1.

My first thought was ANSI NULLS, but setting them on or off made no difference.

This is a simplified example of a larger problem we found in our application (System was checked against a list of numbers – IN(1, 2, etc.)). We replaced this check with a foreign key (instead of IN)and a new check constraint that allowed either, both null or both not null; doing that prevented the third insert.

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, [System]) Values('123', null);

--Bad data stopped
insert TestCheck (Code, [System]) Values(null, 1);
insert TestCheck (Code, [System]) Values('123', 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

Answer 1

The result of evaluating the current constraint for the values 123, NULL is Undefined.

• ([Code] IS NULL AND [System] IS NULL) evaluates to False
• ([Code] IS NOT NULL AND [System] IN (1, 2, 3)) evaluates to Undefined

Result is Undefined

Check Constraint

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

You should change your check for [System] IN (1, 2, 3) to ISNULL([System], 0) IN (1, 2, 3).

Your check constraint then becomes

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, 2, 3))   --Both not null ????
)