Unique Constraint

SQL Unique Constraint

A SQL Unique constraint verifies that no duplicate values are inserted in a specific columns that do not participate in a primary key.
A UNIQUE constraint can be referenced by a FOREIGN KEY constraint.
UNIQUE constraints allow for the value NULL.
When a unique constraint is created it automatically creates a unique index in the database.

Example

CREATE TABLE Certifications
(
ID INT PRIMARY KEY,
NAME VARCHAR(250) NOT NULL,
DESCRIPTION VARCHAR(500),
CONSTRAINT UK_IDCERT UNIQUE(NAME) 
);

Drop Unique Constraint

ALTER TABLE Certifications
DROP CONSTRAINT UK_IDCERT;

Add Unique Constraint

ALTER TABLE Certifications
ADD CONSTRAINT UK_IDCERT UNIQUE (NAME);