Transact-SQL Topics: Unique Values on a Column

	Unique Values on a Column

Description

You may want a column to receive a unique value for each of its records.

To specify that a column will require unique values, when creating it in SQL, use the UNIQUE keyword.

Here is an example:

USE Exercise;
GO
CREATE TABLE Students
(
    StudentNumber int UNIQUE,
    FirstName nvarchar(50),
    LastName nvarchar(50) NOT NULL
);
GO

When a column has been marked as unique, during data entry, the user must provide a unique value for each new record created. If an existing value is assigned to the column, this would produce an error:

USE Exercise;
GO
CREATE TABLE Students
(
    StudentNumber int UNIQUE,
    FirstName nvarchar(50),
    LastName nvarchar(50) NOT NULL
);
GO

INSERT INTO Students
VALUES(24880, N'John', N'Scheels'),
      (92846, N'Rénée', N'Almonds'),
      (47196, N'Peter', N'Sansen'),
      (92846, N'Daly', N'Camara'),
      (36904, N'Peter', N'Sansen');
GO

By the time the fourth record is entered, since it uses a student number that exists already, the database engine would produce an error:

Msg 2627, Level 14, State 1, Line 2
Violation of UNIQUE KEY constraint 'UQ__Students__DD81BF6C145C0A3F'. 
Cannot insert duplicate key in object 'dbo.Students'.
The statement has been terminated.