How Can Filtered Indexes Enforce Conditional Unique Constraints in SQL Server?

Conditional Unique Constraints with Filtered Indexes in SQL Server

Database design sometimes requires conditional unique constraints: uniqueness enforced on a column set only when another column meets a specific condition. This is common when managing active and inactive records, preserving data integrity. SQL Server's filtered indexes offer an efficient solution. A filtered index creates a unique index on a table's subset of rows, defined by a filter predicate.

Implementing Uniqueness Based on Record Status

Imagine a table with ID, Name, and RecordStatus columns. We need a unique constraint on (ID, RecordStatus) only when RecordStatus = 1 (active records). A filtered index achieves this:

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

This ensures uniqueness for ID only when RecordStatus is 1, allowing multiple inactive (RecordStatus = 2) records with the same ID.

Beyond Record Status: Versatile Conditional Uniqueness

Filtered indexes aren't limited to record status. They enable granular control over data uniqueness based on various criteria, enhancing data integrity and performance.

Advantages of Filtered Indexes

Compared to alternatives like triggers, filtered indexes offer:

• Performance Enhancement: Indexing only relevant rows speeds up lookups and inserts.
• Reduced Storage: Less storage is used as the index covers only necessary rows.
• Simplified Maintenance: Automatic updates with data changes eliminate extra maintenance.

Summary

SQL Server's filtered indexes provide a robust and efficient way to enforce conditional unique constraints. They offer a balance between data integrity, performance, and storage optimization.

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