How Can I Use Subqueries in SQL Server Check Constraints?

Using Subqueries in Check Constraints

In SQL Server 2008 R2, attempting to define a check constraint with a subquery, such as:

ALTER TABLE Table1
WITH CHECK ADD CONSTRAINT CK_Code
CHECK (MyField in (Select Field From Table2))

results in an error indicating that subqueries are not allowed in this context.

Alternative Solution: Scalar Function

To achieve this validation without a trigger, a scalar function that encapsulates the subquery can be created and used within the check constraint. Here's an example:

CREATE FUNCTION myFunction(
    @field DATATYPE(?)
)
RETURNS VARCHAR(5)
AS
BEGIN
    IF EXISTS (SELECT * FROM Table2 WHERE MYFIELD = @field)
        RETURN 'True'
    RETURN 'False'
END

Applying the Function to the Check Constraint

Once the function is defined, it can be used in the check constraint as follows:

ALTER TABLE Table1
WITH CHECK ADD CONSTRAINT CK_Code
CHECK (myFunction(MYFIELD) = 'True')

By checking the scalar value returned by the function against a specific string (e.g., 'True'), the desired validation is enforced without the need for a subquery directly in the check constraint.

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