Identify the incorrect statement(s).
2022
Identify the incorrect statement(s).
- A.
A candidate key is minimal set of one or more attributes that, taken collectively, allows us to uniquely identify any entity in the entity set.
- B.
A candidate key for which no proper subset is also a candidate key is called a super key.
- C.
A super key is a set of one or more attributes that, taken collectively, allows us to uniquely identify any entity in the entity set.
- D.
A super key for which no proper subset is also a super key is called a candidate key.
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Correct answer: B
Answer: The incorrect statement is: "A candidate key for which no proper subset is also a candidate key is called a super key."
Key definitions:
A super key is a set of one or more attributes that uniquely identifies any entity in the entity set.
A candidate key is a minimal super key; that is, none of its proper subsets can uniquely identify the entity.
Why the statement is wrong:
The statement confuses the relationship between candidate keys and super keys. It suggests that a candidate key with a minimality property is called a super key. In fact, any candidate key is a super key, but the defining feature of a candidate key is minimality. A super key need not be minimal.
Correct relationship: every candidate key is a super key, and candidate keys are those super keys that are minimal (no proper subset is also a super key).
Counterexample to illustrate: if {SSN} uniquely identifies people then {SSN} is a candidate key. The set {SSN, Name} is a super key but not a candidate key because it has a proper subset {SSN} that is also a super key.
Therefore the only incorrect statement is the one that misstates these definitions; the other statements correctly describe super keys and candidate keys.