Can Scalar-Valued Functions Bypass Subquery Restrictions in SQL Server Check Constraints?

Subquery Functionality in Check Constraints

The use of subqueries in check constraints has been a longstanding limitation in SQL-Server 2008 R2. This article explores the issue and provides an alternative solution using a scalar-returning function.

Problem:

When attempting to validate a column value against another table using a subquery in a check constraint, the following error is encountered: "Sub-queries are not allowed in this context. Only scalar expressions are allowed."

Background:

Check constraints are used to enforce database integrity by evaluating an expression against each row that is inserted or updated in a table. Typically, these expressions consist of scalar operations, not subqueries.

Solution:

To circumvent this limitation, a custom scalar-returning function can be created that encapsulates the necessary subquery. The function should output a scalar value indicating whether the subquery condition is met.

Example:

Suppose we have a table called 'Table1' with a column 'MyField' that needs to be validated against a table called 'Table2'. We can create a function called 'myFunction' as follows:

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:

The 'myFunction' can now be used in the check constraint as follows:

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

This effectively emulates the functionality of the subquery in the check constraint.

Conclusion:

Although subqueries cannot be directly used in check constraints, scalar-returning functions can be employed to achieve the desired validation. This workaround allows for more complex data validation scenarios without compromising database integrity.

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