SQL UNIQUE Constraint

SQL UNIQUE Constraint

The UNIQUE constraint uniquely identifies each record in a database table.

The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.

A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.

you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.

CREATE TABLE Persons
(
P_Id int NOT NULL UNIQUE,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

UNIQUE Constraint on ALTER TABLE
To create a UNIQUE constraint on the "P_Id" column when the table is already created, use the following SQL:

ALTER TABLE Persons
ADD UNIQUE (P_Id)

To DROP a UNIQUE Constraint
To drop a UNIQUE constraint, use the following SQL:
ALTER TABLE Persons
DROP CONSTRAINT uc_PersonID