SQL UNIQUE Constraint
SQL UNIQUE Constraint
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.
SQL UNIQUE Constraint on CREATE TABLE
The following SQL creates a UNIQUE constraint on the "ID" column when the "Persons" table is created:
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)
);To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
CREATE TABLE Persons
(
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
CONSTRAINT UC_Person UNIQUE (ID,LastName)
);SQL UNIQUE Constraint on ALTER TABLE
To create a UNIQUE constraint on the "ID" column when the table is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
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:
MySQL / SQL Server / Oracle / MS Access:
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;
