unique key

Unique key

No unique can not be nulll....unique must have any unique value for unique identification.,...

A field that is not unique and null can not be used as a primary key. Praimary key must satisfy the conditions of not null and unique.

All Primary keys are definitely Candidate Keys. A Candidate key is one which can be used as a Primary key that is not null and unique. That is one of the candidate keys can be chosen as a primary key.A Candidate key is a Unique Key and it can be used to find out any particular Tuple (row) in a table.

The following are the differences between A Candidate key and a Primary Key:

1) A Unique key can be null but not a Primary key

2) On a table we can have only 1 primary key but 'N' number of unique keys.