What is UNIQUE constraint in sql server

UNIQUE

UNIQUE constraint is used for the uniqueness of the value for the column in the table. UNIQUE constraint can have multiple in a table.

Example

Following is the example to apply UNIQUE Constraint on single column in CREATE TABLE

CREATE TABLE Employee

(

Id int NOT NULL UNIQUE,

Name varchar(255) NOT NULL,

Address varchar(255),

Age int,
)

Following is the example to apply UNIQUE Constraint on multiple column in CREATE TABLE

CREATE TABLE Employee

(

Id int NOT NULL UNIQUE,

MobileNO int NULL UNIQUE,

Name varchar(255) NOT NULL,

Address varchar(255),

Age int,
)

Following is the example to apply UNIQUE Constraint on ALTER TABLE

ALTER TABLE Employee

ADD CONSTRAINT chk_ Id UNIQUE

Following is the example to DROP a UNIQUE Constraint

ALTER TABLE Employee

DROP CONSTRAINT chk_ Id