


How to Enforce a Unique Constraint Across Multiple Columns in SQL Server?
Enforcing Uniqueness Across Multiple SQL Server Columns
Often, database integrity requires preventing duplicate rows based on multiple column values. This article demonstrates how to enforce a unique constraint across the PersonNumber
and Active
columns in a Person
table.
Implementing the Unique Constraint
After removing any pre-existing duplicate entries, apply the constraint using one of these methods:
-
ALTER TABLE
Statement: This directly adds the constraint to the existing table.ALTER TABLE dbo.Person ADD CONSTRAINT uq_Person_NumberActive UNIQUE (PersonNumber, Active);
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CREATE UNIQUE INDEX
Statement: This creates a unique index, which implicitly enforces the uniqueness constraint.CREATE UNIQUE INDEX uq_Person_NumberActive ON dbo.Person (PersonNumber, Active);
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Alternative Approaches & Optimization
While the above methods are effective, consider these enhancements:
- Performance: Proactive duplicate checks before attempting an insert can improve performance by avoiding exceptions within
TRY...CATCH
blocks. INSTEAD OF
Trigger: AnINSTEAD OF
trigger allows for conditional insert logic, preventing exceptions from propagating to the application layer. This offers a more robust error-handling mechanism.
Illustrative Example
This example demonstrates the unique constraint in action:
-- Create the Person table CREATE TABLE dbo.Person ( ID INT IDENTITY(1, 1) PRIMARY KEY, Name NVARCHAR(32) NOT NULL, Active BIT NOT NULL DEFAULT 0, PersonNumber INT NOT NULL ); -- Add the unique constraint ALTER TABLE dbo.Person ADD CONSTRAINT uq_Person_NumberActive UNIQUE (PersonNumber, Active); -- Insert data INSERT INTO dbo.Person (Name, Active, PersonNumber) VALUES ('John Doe', 1, 1234); INSERT INTO dbo.Person (Name, Active, PersonNumber) VALUES ('Jane Doe', 0, 5678); -- Attempt to insert a duplicate (this will fail) INSERT INTO dbo.Person (Name, Active, PersonNumber) VALUES ('John Doe', 1, 1234);
This ensures that only one record exists for each unique combination of PersonNumber
and Active
status. Any attempt to insert a duplicate will result in an error. Remember to replace dbo.Person
with your actual schema and table name.
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