A unique index is an index that uniquely identify a row. A unique index constraint prevents duplicate values from being entered into a table, and can prevent null values, if defined. The differences between a unique key and a unique index is that the unique key must be defined a Not NULL.
Notes:
•A Unique Index constraint can replace a "Require a unique value" rule.
•A Unique Index constraint can replace a "Require a value" rule.
•A Unique Index constraint automatically builds an index.
