How Can I Efficiently Enforce Unique Constraints on SQL Server Columns Allowing NULLs?

Specialized Indexing for Unique Constraints on Null Columns

In SQL Server database management, the question arises regarding the creation of unique indexes on columns that allow NULL values. Traditional methods often involve creating views with filters to enforce uniqueness while accommodating NULLs. However, alternative approaches provide more efficient solutions.

Filtered Indexes for Selective Uniqueness Constraint Enforcement

SQL Server 2008 and later versions introduced filtered indexes, which allow the creation of unique indexes on specific subsets of data. This feature proves particularly useful for handling unique constraints on columns that admit NULL values.

To create a filtered index for a unique constraint on a column with NULLs permitted, follow the syntax:

CREATE UNIQUE INDEX AK_MyTable_Column1 ON MyTable (Column1) WHERE Column1 IS NOT NULL;

The WHERE clause in the index definition limits the scope of the uniqueness constraint to only non-NULL values in the column. This ensures that uniqueness is enforced while allowing NULL values to exist in the table.

Trigger-Based Uniqueness Checking

An alternative method involves the creation of a trigger on the table to check for uniqueness before inserting or updating data. This trigger can verify that no duplicate unique values are inserted, maintaining the uniqueness constraint.

The following trigger example can be used:

CREATE TRIGGER TR_MyTable_Unique
ON MyTable
FOR INSERT, UPDATE
AS
BEGIN
    IF EXISTS (
        SELECT *
        FROM MyTable
        WHERE Column1 = NEW.Column1
        AND Column1 IS NOT NULL
    )
    BEGIN
        RAISERROR('Duplicate unique value detected.', 16, 1);
    END;
END;

While triggers provide a viable alternative, they can introduce performance overhead, especially with high levels of data insertion or updates.

In conclusion, filtered indexes present a more efficient and selective approach to implementing unique constraints on columns with NULL values. However, trigger-based uniqueness checks can serve as a viable option in environments where filtered indexes are not supported or when additional validation logic is required.

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