A unique key is a column or set of columns that uniquely identify a row; in other words, each value in a unique key column is unique. A unique key constraint prevents duplicate (non-unique) and null values from being entered into a table. The only difference between a unique key and a primary key is that you can define multiple unique keys per table.
Notes:
•A Unique Key constraint can replace a "Require a unique value" rule.
•A Unique Key constraint automatically builds an index.
