Why can a candidate key be null?


Candidate key is the set of attributes by which it is possible to identify each row of the table. Therefore, if some attribute is nullable, it cannot be one of candidate keys.

Can a candidate key have a null value?

Any attribute of Primary key can not contain NULL value. While in Candidate key any attribute can contain NULL value.

Can any candidate key be a primary key?

Note that each candidate key can be a primary key, but only one should be chosen as the primary key. The rule to become a primary key among candidate keys is that the key’s attribute values must be unique and can never be Null for any domain.

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