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The UNIQUE constraint ensures that all values in a column are different
The UNIQUE
constraint ensures that all values in a column are different.
Both the UNIQUE
and
PRIMARY KEY
constraints provide a guarantee for uniqueness for a column or set of columns.
A PRIMARY KEY
constraint automatically has a
UNIQUE
constraint.
However, you can have many UNIQUE
constraints per table, but only one
PRIMARY KEY
constraint per table.
The following SQL creates a UNIQUE
constraint on the "ID" column when the "Persons" table is created:
CREATE TABLE Persons
(
ID int NOT NULL UNIQUE,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int
);
CREATE TABLE Persons
(
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
UNIQUE (ID)
);
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)
);
To create a UNIQUE
constraint on the "ID" column when the table is already created, use the following SQL:
ALTER TABLE Persons
ADD UNIQUE (ID);
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);
To drop a UNIQUE
constraint, use the following SQL:
ALTER TABLE Persons
DROP INDEX UC_Person;
ALTER TABLE Persons
DROP CONSTRAINT UC_Person;