Unique indexes

A unique index is similar to a primary key in that it will not allow duplicate values in the field for which it has been set. However, it is unlike a primary key in that you can set as many unique indexes as you wish for a table, and you are allowed to leave a unique index field blank for any record. This is obviously useful in that you may not want duplicate data to be allowable in a particular field, though you do want to allow certain records to be blank in this field. A unique index field, unlike a primary key field, will not count two or more blanks as duplicates.

See:

Creating indexes and primary keys.