SQL SERVER UNIQUE KEY Constraint

1	The UNIQUE constraint uniquely identifies each record in a database table.
2	The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.
3	A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it. Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.

 

 

Example :

 

CREATE TABLE Persons

(

P_Id int NOT NULL UNIQUE,

LastName varchar(255) NOT NULL,

FirstName varchar(255),

Address varchar(255),

City varchar(255)

)

 

To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use the following SQL syntax:

 

CREATE TABLE Persons

(

P_Id int NOT NULL,

LastName varchar(255) NOT NULL,

FirstName varchar(255),

Address varchar(255),

City varchar(255),

CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)

)