This is the questions and answers section on "DBMS Keys" with explanation for various interview, competitive examination and entrance test. Solved examples with detailed answer description, explanation are given and it would be easy to understand.
1. Which one of the following is a set of one or more attributes taken collectively to uniquely identify a record?
- A. Candidate key
- B. Sub key
- C. Super key
- D. Foreign key
Answer: Option C
Explanation:
Super key
Super key is the superset of all the keys in a relation.
2. Consider attributes ID , CITY and NAME . Which one of this can be considered as a super key ?
Answer: Option B
Explanation:
ID
Here the id is the only attribute which can be taken as a key. Other attributes are not uniquely identified .
3. The subset of super key is a candidate key under what condition ?
- A. No proper subset is a super key
- B. All subsets are super keys
- C. Subset is a super key
- D. Each subset is a super key
Answer: Option A
Explanation:
No proper subset is a super key
The subset of a set cannot be the same set.Candidate key is a set from a super key which cannot be the whole of the super set
4. A _____ is a property of the entire relation, rather than of the individual tuples in which each tuple is unique.
Answer: Option B
Explanation:
Key
Key is the constraint which specifies uniqueness.
5. Which one of the following attribute can be taken as a primary key ?
Answer: Option C
Explanation:
Id
The attributes name , street and department can repeat for some tuples.But the id attribute has to be unique .So it forms a primary key.
