General doubt dbms

Question

An instance of a relational schema R(A,B,C,D) has distinct values for attribute A then A can be

1) candidate key

2)non key

3)super key

4)none

Answer 1

Any attribute that can be used to uniquely identify a row can be a super key and a minimal super key is called as candidate key.

Since the key here has only one attribute, it is a candidate key.

Comments

why not it can be NON-KEY?

think at the instance it is giving different values, may be after some time ( i mean at some other instance ), it have duplicates.

---

but they have asked for that particular instance .. which should be candidate key only...

---

I think for the given particular instance, A can be used as a candidate key. Of course, it might refer to duplicates later, but we don't know about that, so we can't comment on that.

At least that's how I interpreted this question.

---

i too notice hat, But keys and Functional Dependencies can't be said to Particular instances.

By knowing Keys and Functional Dependencies, we have to take instances.

---

for a given instance, it should be a foreign key.

---

@shaikh sir you are right

Answer 2

Here ,we may say that A->BCD ,FD holds for this instance
And $A^+={A,B,C,D}$

But FD s are defined on the schema itself not the instance, so based on the state of the instance
we cannot say what holds for schema (there can be a many instances for R).