Which one is correct w.r.t. RDBMS ?
2017
Which one is correct w.r.t. RDBMS ?
- A.
primary key ⊆ super key ⊆ candidate key
- B.
primary key ⊆ candidate key ⊆ super key
- C.
super key ⊆ candidate key ⊆ primary key
- D.
super key ⊆ primary key ⊆ candidate key
Attempted by 1092 students.
Show answer & explanation
Correct answer: B
Correct relationship: primary key ⊆ candidate key ⊆ super key
Superkey: A set of attributes that uniquely identifies a tuple in a relation. A superkey may contain extra attributes that are not necessary for uniqueness.
Candidate key: A minimal superkey, i.e., a superkey with no proper subset that is also a superkey.
Primary key: A single candidate key chosen to uniquely identify tuples in a relation.
Explanation: Because a primary key is one chosen candidate key, and every candidate key is by definition a superkey, the correct inclusion order is primary key ⊆ candidate key ⊆ superkey. Equality can hold between primary key and some candidate key since the primary key is exactly one candidate key.
Example: For a relation Student(ID, Name, Email), if ID uniquely identifies students, {ID} is a candidate key (and may be chosen as the primary key). A set like {ID, Email} is a superkey because it also uniquely identifies tuples but is not minimal.