Prime attribute

Question

Primary key cannot be NULL . But is there any constraint on prime attributes also?

Can they be NULL?

Answer 1

What are prime attributes?

That are part of some candidate key.

Candidate key has to be unique and minimal,there is not restriction of null and not null on candidate key.It may or may not null.

But when i choose one primary key from candidate keys(In case multiple candidate keys),then i have to choose the one which  is not null because it is primary key definition(Non null candidate key).

Now remaining candidate keys becomes the alternate keys .And alternate key can be Null.Alternate key subset can be null.

So,which means prime attributes can be Null.

More strict can be :- If a prime attribute is part of primary key,then it can't be Null.Otherwise it may or may not Null

Comments

Thank you so much