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SQL – UNIQUE Key

SQL UNIQUE Key Constraint :

1 . At The Time Of Table Creation :

UNIQUE Key Constraint is used to identify every record uniquely. Multiple Unique Key can be in a sigle table.

SQL – UNIQUE Key Constraint Syntax Example :

Example

CREATE TABLE UserProfile (
ID int(11) NOT NULL,
Name varchar(110) NOT NULL,
Email varchar(100) NOT NULL,
CONSTRAINT USER_EMAIL_UNIQUE_CONSTRAINT UNIQUE (Email)
) ;

Where USER_EMAIL is Constraint’s name you can choose constraint name as per you understanding.

2 .In Existing Table :

Example

ALTER TABLE UserProfile
ADD CONSTRAINT USER_EMAIL_UNIQUE_CONSTRAINT UNIQUE (Email);

Drop UNIQUE Key Constraint :

Example

ALTER TABLE UserProfile
DROP CONSTRAINT USER_EMAIL_UNIQUE_CONSTRAINT;

Note : In Mysql You have to Use Following Query to Drop the Constraint :

Example

ALTER TABLE UserProfile
DROP INDEX USER_EMAIL_UNIQUE_CONSTRAINT;