SQL UNIQUE Constraint

SQL UNIQUE Constraint:

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

UNIQUE constraints provide a guarantee for uniqueness for a column or set of columns.

You can have many UNIQUE constraints per table.

SQL UNIQUE Constraint on CREATE TABLE:

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

UNIQUE constraint on multiple columns:

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:

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

DROP a UNIQUE Constraint:

ALTER TABLE Persons DROP INDEX UC_Person;