Crafting Unique Data Integrity in SQL Server 2005: A Guide to Unique Constraints

• SQL (Structured Query Language): It's a language for interacting with relational databases like SQL Server. It allows you to create, manipulate, and retrieve data.
• SQL Server: This is a specific relational database management system (RDBMS) developed by Microsoft.
• Constraints: These are rules that define limitations on the data stored in a table. They help ensure data integrity and consistency.

Creating a Unique Constraint in SQL Server 2005:

There are two main ways to create a unique constraint:

1. Using Transact-SQL (T-SQL) code: This involves writing a CREATE TABLE statement with a CONSTRAINT clause. Here's an example:

CREATE TABLE Customers (
  CustomerID int PRIMARY KEY,
  CustomerName varchar(50) NOT NULL,
  Email nvarchar(255) UNIQUE
);

In this example, the Email column has a unique constraint, meaning no two customers can have the same email address.

1. Using SQL Server Management Studio (SSMS): This is a graphical tool for managing SQL Server databases. You can create a unique constraint through the table designer. Here's a summary of the steps:

   • Right-click on the table in Object Explorer.
   • Select Design.
   • Go to Indexes/Keys.
   • Click Add.
   • Choose the column(s) for the constraint.
   • Set Type to Unique.
   • Give the constraint a name.
   • Save the table.

CREATE TABLE Products (
  ProductID int PRIMARY KEY,
  ProductName varchar(50) NOT NULL,
  CHECK (EXISTS (SELECT 1 FROM Products p2 WHERE p2.ProductID <> ProductID AND p2.ProductName <> ProductName))
);

This code defines a CHECK constraint that verifies if there's another product with the same name before allowing the insert operation. However, it requires an additional query for each insert, impacting performance compared to a dedicated unique constraint.

Using Unique Nonclustered Indexes:

This method creates a unique nonclustered index on the desired column(s). While indexes don't directly enforce uniqueness through constraints, attempting to insert duplicate values will lead to an index violation error.

CREATE TABLE Orders (
  OrderID int PRIMARY KEY,
  CustomerID int NOT NULL,
  OrderDate date NOT NULL,
  UNIQUE NONCLUSTERED INDEX IX_UniqueOrder (CustomerID, OrderDate)
);

This approach offers performance benefits for frequently used query patterns involving the unique column(s) but doesn't prevent duplicate data entry attempts as effectively as a dedicated unique constraint.