使用IN条件对新表的检查约束未按预期工作

Question

I have created a new table that included a check constraint using an in clause to create a list of allowed entries, or at least that was the plan. The create table command executed without error however in testing I am able to insert rows without the constraint preventing incorrect values for the column "CAUSE"

I have done a bit of internet reasearch but I have not figured this one our yet. I will include the create table statement bellow.

'''

USE TEST 
GO

CREATE TABLE EMP_POINTS (
ROWID int NOT NULL IDENTITY(1,1),
WEEK_ID nvarchar(15) NOT NULL,
EMPLOYEE_ID nvarchar(15) NOT NULL,
DATE date NOT NULL,
CAUSE nvarchar(15),
POINTS decimal(2,1),

PRIMARY KEY(EMPLOYEE_ID, DATE),
CONSTRAINT FK_EmpID FOREIGN KEY(EMPLOYEE_ID) REFERENCES EMPLOYEE(ID),
CONSTRAINT chk_cause CHECK (
                            CAUSE IN('Late In','Early Out','E & L', 'NCNS', 'Absent', '0.5','1.0', NULL)
                           )
);

'''

I would expect that the only values which would be allowed in the CAUSE column would be the ones identified in the chk_cause constraint however it is allowing any values to be entered.

Answer 1

CHECK constraints are different from WHERE and CASE conditions. A CHECK constraint only fails when the result is explicitly false. The others require that the condition is explicitly true. The difference is how NULL values are handled.

So, you can write your constraint as:

CONSTRAINT chk_cause CHECK (CAUSE IN('Late In', 'Early Out', 'E & L', 'NCNS', 'Absent', '0.5', '1.0')
                           )

Then, only these values and NULL will be allowed (assuming CAUSE is not declared NOT NULL ).

The problem with your version is that other values will result in NULL (due to the NULL in the IN list). And that passes the CHECK constraint.