How Can I Implement Conditional Unique Constraints in SQL Server 2005?

SQL Server 2005: Implementing Conditional Uniqueness for Column Subsets

Creating unique constraints that apply only under specific conditions in SQL Server 2005 presents a challenge. While triggers offer a solution, they can impact performance. A more efficient approach utilizes filtered indexes.

Filtered Indexes: A Dynamic Solution for Conditional Constraints

Filtered indexes allow indexing of specific data subsets, providing a powerful mechanism for implementing conditional constraints through filter predicates.

Creating a Conditional Unique Constraint using a Filtered Index

The following T-SQL statement demonstrates how to create a unique index with a filter:

CREATE UNIQUE INDEX MyIndex
ON MyTable (ID)
WHERE RecordStatus = 1;

This creates a unique constraint on the ID column, but only when RecordStatus is 1. Any attempt to violate this conditional uniqueness will generate an error.

Example Error Message:

Attempting to insert a duplicate ID value when RecordStatus is 1 will result in:

<code>Msg 2601, Level 14, State 1, Line 13
Cannot insert duplicate key row in object 'dbo.MyTable' with unique index 'MyIndex'. The duplicate key value is (9999).</code>

Summary

Filtered indexes offer an efficient and elegant method for establishing conditional unique constraints in SQL Server 2005, avoiding the performance overhead often associated with trigger-based solutions. The use of filter predicates allows for dynamic control over uniqueness within specific data subsets.

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