Difference between primary key and candidate key

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1.	Difference between primary key and candidate key
Answer»		Primary Key	Candidate Key	Primary key is a minimal super key. So there is one and only one primary key in a relation.	While in a relation there can be more than one candidate key.	Any attribute of Primary key can not contain NULL value.	While in Candidate key any attribute can contain NULL value.	Primary key can be optional to specify any relation.	But without candidate key there can’t be specified any relation.	Primary key specifies the important attribute for the relation.	Candidate specifies the key which can qualify for primary key.	Its confirmed that a primary key is a candidate key.	But Its not confirmed that a candidate key can be a primary key.

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