SQL UNIQUE Constraint

The UNIQUE constraint uniquely identifies each record in a database table.

The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.

A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.

Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.

CREATE TABLE Persons
(
xx_xx int NOT NULL,
xx varchar(255) NOT NULL,
xxxx varchar(255),
xx varchar(255),
xxx varchar(255),
CONSTRAINT xx_xx UNIQUE (x_xx,xxxx)
)

ALTER TABLE xxx
ADD CONSTRAINT uc_PersonID UNIQUE (xxx_xxx,xxx)