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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.

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Coattail effect