How Can I Use Subqueries in SQL Server 2008 R2 Check Constraints?

Subqueries in Check Constraints: A Workaround

In SQL Server 2008 R2, using subqueries in check constraints triggers an error message stating that only scalar expressions are allowed. For example, the following code aims to check a column in Table1 against values in Table2 when inserting data:

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

Addressing the Error

To circumvent this limitation, consider alternative approaches:

1. Using a Function:

Convert the subquery into a function that returns a scalar value (e.g., True or False) indicating whether the value exists in the other table. Then, use the function in the check constraint.

For instance:

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

2. Alternate Syntax:

In some cases, it's possible to rewrite the check constraint using alternate syntax that avoids subqueries:

• Using EXISTS:

  CHECK (EXISTS (SELECT* FROM Table2 WHERE MYFIELD = MyField))

• Using IN with a subselect:

  CHECK (MyField IN (SELECT Field FROM Table2))

3. Foreign Key Constraint:

If the relationship between the tables represents a true foreign key relationship, the most appropriate solution is to use a foreign key constraint. This enforces referential integrity and guarantees data consistency.

By implementing one of the mentioned workarounds, you can utilize subquery-like functionality in check constraints, albeit with additional considerations such as implementing functions or using alternate syntax.

The above is the detailed content of How Can I Use Subqueries in SQL Server 2008 R2 Check Constraints?. For more information, please follow other related articles on the PHP Chinese website!