Microsoft® SQL Server 2012 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 only 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 ...