Microsoft® SQL Server 2008 R2 Unleashed

The UNIQUE Constraint

The UNIQUE constraint is functionally similar to PRIMARY KEY. It also uses a unique index to enforce uniqueness, but unlike PRIMARY KEY, it allows nulls in the columns that participate in the UNIQUE constraint. The definition of a UNIQUE constraint with columns that are nulls is generally impractical. The value of NULL is considered a unique value, so you are limited to the number of rows that can be inserted with NULL values. For example, only one row with a NULL value in the constraint column can be inserted if the UNIQUE constraint is based on a single column. UNIQUE constraints with multiple nullable columns can have more than one row with null values in the constraint keys, but the number of rows is limited to the ...