The UNIQUE constraint ensures that all values in a column are unique.
Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns. However, you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.
UNIQUE Constraint on CREATE TABLE
The following SQL defines a UNIQUE constraint for the "ID" column upon creation of the "Persons" table:
SQL Server / Oracle / MS Access:
CREATE TABLE Persons
(
ID int NOT NULL UNIQUE,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int
);MySQL
CREATE TABLE Persons
(
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
UNIQUE (ID)
);Naming a Unique Constraint
To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:
CREATE TABLE Persons
(
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
CONSTRAINT UC_Person UNIQUE (ID,LastName)
);UNIQUE Constraint on ALTER TABLE
To create a UNIQUE constraint on the "ID" column when the table is already created, use the following SQL syntax:
ALTER TABLE Persons
ADD UNIQUE (ID);Naming a Unique Constraint
To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:
ALTER TABLE Persons
ADD CONSTRAINT UC_Person UNIQUE (ID,LastName);Drop a UNIQUE Constraint
To drop a UNIQUE constraint, use the following SQL:
MySQL
ALTER TABLE Persons
DROP INDEX UC_Person;SQL Server / Oracle / MS Access:
ALTER TABLE Persons
DROP CONSTRAINT UC_Person;