UNIQUE KEY

---

UNIQUE KEY in SQL Server Database Table

A unique key is a set of one or more than one fields/columns of a table that uniquely identify a record in a database table.

* The unique key and primary key both provide a guarantee for uniqueness for a column or a set of columns.

* There is an automatically defined unique key constraint within a primary key constraint.

* There may be many unique key constraints for one table, but only one PRIMARY KEY constraint for one table.

SQL UNIQUE KEY constraint on CREATE TABLE-(Defining a unique key constraint on single column):

Syntax

CREATE TABLE students  
(  
S_Id int NOT NULL UNIQUE,  
LastName varchar (255) NOT NULL,  
FirstName varchar (255),  
City varchar (255)  
)  

(Defining a unique key constraint on multiple columns):

 CREATE TABLE students  
(  
S_Id int NOT NULL,  
LastName varchar (255) NOT NULL,  
FirstName varchar (255),  
City varchar (255),  
CONSTRAINT uc_studentId UNIQUE (S_Id, LastName)  
)  

SQL UNIQUE KEY constraint on ALTER TABLE-(Defining a unique key constraint on single column)

Syntax

ALTER TABLE students  
ADD UNIQUE (S_Id)  

(Defining a unique key constraint on multiple columns):

ALTER TABLE students  
ADD CONSTRAINT uc_StudentId UNIQUE  (S_Id, LastName)