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 ...