Ans. UNIQUE KEY constraint requires a unique value throughout the named column (or combinations of the columns) in the table. Unlike the primary key a UNIQUE KEY constraint does not automatically prevents you from having a NULL value. Whether NULLs are allowed or not depends on how you set the NULL option for that column in the table. Keep in mind, however, that, if you do allow NULLs, you will able to insert only one of them.