MySQL UNIQUE

MySQL UNIQUE Constraint

The UNIQUE constraint ensures that all values in a column are unique.

Both UNIQUE and PRIMARY KEY constraints enforce uniqueness for a column or a combination of columns.

A PRIMARY KEY constraint inherently includes a UNIQUE constraint.

While multiple UNIQUE constraints can exist per table, only one PRIMARY KEY constraint can be defined per table.

UNIQUE Constraint on CREATE TABLE

The following SQL statement establishes a UNIQUE constraint on the “ID” column during the creation of the “Persons” table:

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 for 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 add a UNIQUE constraint to the “ID” column in an existing table, use the following SQL:

ALTER TABLE Persons
ADD UNIQUE (ID);

To assign a name to a UNIQUE constraint and define it for multiple columns, use the following SQL syntax:

ALTER TABLE Persons
ADD CONSTRAINT UC_Person UNIQUE (ID,LastName);

DROP a UNIQUE Constraint

To remove a UNIQUE constraint, use the following SQL:

ALTER TABLE Persons
DROP INDEX UC_Person;