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

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.

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

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@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).