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SQL UNIQUE

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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.

UNIQUE on CREATE TABLE

The following SQL creates a UNIQUE constraint on the "ID" column when the "Persons" table is created:

UNIQUE on CREATE TABLE Example

CREATE TABLE Persons (
    ID int NOT NULL UNIQUE,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int
);

To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following syntax:

Name a UNIQUE Constraint Example

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    CONSTRAINT UC_Person UNIQUE (ID, LastName)
);

UNIQUE on ALTER TABLE

To create a UNIQUE constraint on the "ID" column after the table is created, use the following SQL:

ALTER TABLE Persons
ADD UNIQUE (ID);
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