How Can I Create a Unique Index on a Column Allowing NULL Values in SQL Server?

Creating Unique Indices on Null Columns

Despite the common misconception that unique constraints cannot be applied to columns allowing NULL values, there are indeed methods to achieve this in SQL Server 2005.

Approach 1: View with Unique Index

The approach mentioned in the question utilizes a view to exclude NULLs and create a unique index on that view. While effective, this solution adds complexity and performance overhead.

Approach 2: Filtered Index (SQL Server 2008 )

Introducing SQL Server 2008, filtered indices allow you to specify a WHERE clause when creating the index. This enables the creation of unique indices on columns that may contain NULLs.

Syntax:

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

Approach 3: Trigger-Based Enforcement

Another alternative is to implement a trigger that checks for uniqueness whenever a row is inserted or updated. However, triggers can impact performance, especially on tables with high insert/update rates.

Implementation:

CREATE TRIGGER MyTable_TRG_Unique ON MyTable
AFTER INSERT OR UPDATE
AS
BEGIN
  IF EXISTS(SELECT 1 FROM MyTable WHERE Column1 = (SELECT Column1 FROM INSERTED))
  BEGIN
    RAISERROR('Column1 must be unique.', 16, 1); -- Handle error as needed
  END
END

Consider the pros and cons of each approach based on the specific requirements and performance constraints of your application.

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