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What is Unique Key in SQL Server?

A unique key in SQL Server is a constraint that ensures the uniqueness of values within one or more columns in a table. It guarantees that each value in the specified column(s) is unique and cannot be duplicated within the table. Let's explain with an example:

Suppose we have a table called "Employees" with the following columns: EmployeeID, FirstName, LastName, and Email. We want to ensure that each employee's email address is unique within the table. To achieve this, we can define a unique key constraint on the "Email" column.

Here's an example of creating a unique key constraint in SQL Server:


CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    FirstName VARCHAR(50),
    LastName VARCHAR(50),
    Email VARCHAR(100) UNIQUE
);

In this example, the "Email" column is defined as a unique key using the UNIQUE keyword. This means that each value in the "Email" column must be unique, and duplicate values are not allowed.

Let's insert some sample data into the "Employees" table to see how the unique key constraint works:


INSERT INTO Employees (EmployeeID, FirstName, LastName, Email)
VALUES (1, 'John', 'Doe', 'john.doe@example.com');

-- This insert will succeed since the email is unique
INSERT INTO Employees (EmployeeID, FirstName, LastName, Email)
VALUES (2, 'Jane', 'Smith', 'jane.smith@example.com');

-- This insert will fail since the email 'john.doe@example.com' already exists
INSERT INTO Employees (EmployeeID, FirstName, LastName, Email)
VALUES (3, 'Mike', 'Johnson', 'john.doe@example.com');

In the above example, the first two inserts succeed because they have unique email addresses. However, the third insert fails because it violates the unique key constraint. It tries to insert an email address that already exists in the table.

By enforcing a unique key constraint on the "Email" column, we can ensure that each employee has a unique email address within the "Employees" table, maintaining data integrity and preventing duplicate entries.

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