UNIQUE

The UNIQUE constraint guarantees uniqueness for all values within a column.

SQL UNIQUE Constraint on CREATE TABLE

The subsequent SQL establishes a UNIQUE constraint on the “ID” column during the 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)
);

To name a UNIQUE constraint and define it on multiple columns, employ 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 add a UNIQUE constraint on the “ID” column after 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 define it on multiple columns, employ 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 remove a UNIQUE constraint, utilize the following SQL:

MySQL:

ALTER TABLE Persons
DROP INDEX UC_Person;

SQL Server / Oracle / MS Access:

ALTER TABLE Persons
DROP CONSTRAINT UC_Person;