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UNIQUE Constraints

SQL Server 2000

UNIQUE Constraints

You can use UNIQUE constraints to ensure that no duplicate values are entered in specific columns that do not participate in a primary key. Although both a UNIQUE constraint and a PRIMARY KEY constraint enforce uniqueness, use a UNIQUE constraint instead of a PRIMARY KEY constraint when you want to enforce the uniqueness of a column, or combination of columns, that is not the primary key.

Multiple UNIQUE constraints can be defined on a table, whereas only one PRIMARY KEY constraint can be defined on a table.

Also, unlike PRIMARY KEY constraints, UNIQUE constraints allow the value NULL. However, as with any value participating in a UNIQUE constraint, only one NULL value is allowed per column.

A UNIQUE constraint can also be referenced by a FOREIGN KEY constraint.

See Also

Creating and Modifying UNIQUE Constraints

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