SQL UNIQUE

SQL UNIQUE Constraint

The UNIQUE constraint guarantees that all values in a column are distinct.

Both UNIQUE and PRIMARY KEY constraints ensure uniqueness for a column or set of columns.

A PRIMARY KEY constraint inherently includes a UNIQUE constraint.

While multiple UNIQUE constraints are permissible per table, only one PRIMARY KEY constraint is allowed per table.

SQL UNIQUE Constraint on CREATE TABLE

The provided 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 assign a name to a UNIQUE constraint and define it across multiple columns, utilize 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 implement a UNIQUE constraint on the “ID” column after the table has been created, employ the following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD UNIQUE (ID);

To assign a name to a UNIQUE constraint and specify it across multiple columns, utilize 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, employ the following SQL:

MySQL:

ALTER TABLE Persons
DROP INDEX UC_Person;

SQL Server / Oracle / MS Access:

ALTER TABLE Persons
DROP CONSTRAINT UC_Person;