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Saturday, 24 September 2016

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

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