Self Doubt

Question

Primary key is subset or proper subset of superkey ?

Comments

I think subset would be more appropriate as it may be equal to or proper subset of superkey .

Answer 1

Depends on the candidate key.

Primary key is selected one among the candidate key(s).

If there is only one candidate key, then that key is itself primary key. Whereas if there are multiple candidate keys, one among them is chosen to be a primary key and the others are referred to as alternate keys.

Now, a candidate key can be a proper subset of super-key or the super-key itself (when there is no proper subset).

Comments

ok so candidate key is proper subset of super key and one of the candidate key leads to primary key, so we can conclude that primary key is also a proper subset of super key.. correct ?

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Not always candidate key is a proper subset of super key. Suppose if the super-key is a simple key, then it will be a candidate key as well as a super-key.

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ok got your point, thanks :)

Answer 2

primary key is subset of superkey

Comments

@ G Shaheena no its not possible always.

Answer 3

PK is always a SK but SK is not always PK.