TSQL - Unique Index vs Unique Constraint! 🤔

When to use a unique index

A unique index also prevents duplicates but is more flexible and often used for performance optimization.

CREATE UNIQUE INDEX IX_Customer_Product
ON Orders (CustomerID, ProductID);        

Why choose a unique index?

• You need indexing for query performance:

If the uniqueness requirement is secondary, but you also need to optimize queries involving the fields, a UNIQUE INDEX might be better.

Example: If frequent searches are performed on CustomerID and ProductID, you can define:

• You need a filtered index. You can create filtered indexes, like enforcing uniqueness only for active rows.

CREATE UNIQUE INDEX IX_Customer_Product ON Orders (CustomerID, ProductID) WHERE Quantity > 0;        

• You need more flexibility (e.g., Ignoring NULL Values in Some DBs):

Some databases allow multiple NULL values in a UNIQUE INDEX, but constraints typically enforce strict uniqueness.

CREATE UNIQUE INDEX IX_Email ON Users (Email) WHERE Email IS NOT NULL;        

This allows multiple users without an email while ensuring uniqueness for those who have one.

The creation of an index should always be carefully evaluated, as indexes need to be maintained with every operation. While they can significantly improve query performance, they also introduce overhead during insert, update, and delete operations, as the database must continuously update the index structure. Poorly planned indexing can lead to increased storage consumption, slower write performance, and even potential deadlocks in high-concurrency environments. Therefore, it's essential to balance the benefits of faster reads with the costs of maintaining indexes.

When to use a unique constraint

A unique constraint is part of the table schema and enforces a business rule directly. It’s ideal when the uniqueness is fundamental to your data model.

For example, imagine you want to prevent a customer from ordering the same product more than once. You can define the table like this:

CREATE TABLE Orders (
    OrderID INT IDENTITY(1,1) PRIMARY KEY,
    CustomerID INT NOT NULL,
    ProductID INT NOT NULL,
    OrderDate DATETIME NOT NULL DEFAULT GETDATE(),
    Quantity INT NOT NULL CHECK (Quantity > 0),

    -- Ensuring no duplicate orders for the same product
    CONSTRAINT UQ_Customer_Product UNIQUE (CustomerID, ProductID)
);        

Why choose a unique constraint?

• Ensuring data integrity as a business Rule: It reflects a business rule and ensures your schema is consistent. If the uniqueness requirement is a core business rule (e.g., "A customer cannot order the same product twice"), then a UNIQUE constraint is the best choice. It is declarative and automatically enforced by the database.
• The uniqueness rule is part of the schema design: If the uniqueness of the columns is essential to the table's logic, a constraint makes it clear and enforceable without additional indexing concerns.
• You want automatic error messages for violations: If you try to insert duplicate values, the database will throw an explicit constraint violation error, making debugging easier.
• Simpler maintenance: Constraints are easier to understand and maintain than manually created indexes.

Which one should you choose?

• If you want to enforce a business rule and prioritize data integrity → Use a unique constraint.
• If you need to optimize query performance while ensuring uniqueness → Use a unique index.

While both options achieve the same goal, the unique constraint is better for logical integrity, is the way to go when the rule is central to your data model. It keeps your schema clean, clear, and enforceable without additional complexity while a unique index is more suited for query performance optimization.

What do you usually use in your projects? Let’s discuss below! 👇

#DatabaseDesign #SQLTips #DataIntegrity #PerformanceOptimization