GATE CS 2014 Set 2 Q.32 (CS Q.22): ER Model and Relational Schema Design

Given an instance of the STUDENTS relation as shown below:

StudentID	StudentName	StudentEmail	StudentAge	CPI
2345	Shankar	shankar@math	X	9.4
1287	Swati	swati@ee	19	9.5
7853	Shankar	shankar@cse	19	9.4
9876	Swati	swati@mech	18	9.3
8765	Ganesh	ganesh@civil	19	8.7

For (StudentName, StudentAge) to be a key for this instance, the value X should NOT be equal to _____.

Your answer

Official answer and solution

Solution

Apply the candidate key uniqueness constraint on the composite key attribute pair (StudentName, StudentAge).

Key Condition:
For (StudentName, StudentAge) to be a key for the relation instance, every tuple in the relation must have a unique pair of values for (StudentName, StudentAge).
Existing Tuples:
- Tuple 1: (StudentID 2345): StudentName = Shankar, StudentAge = X
- Tuple 2: (StudentID 1287): StudentName = Swati, StudentAge = 19
- Tuple 3: (StudentID 7853): StudentName = Shankar, StudentAge = 19
- Tuple 4: (StudentID 9876): StudentName = Swati, StudentAge = 18
- Tuple 5: (StudentID 8765): StudentName = Ganesh, StudentAge = 19
Uniqueness Requirement for Tuple 1:
Tuple 3 already has StudentName = Shankar and StudentAge = 19.
If $X = 19$ , Tuple 1 would have (Shankar, 19), which duplicates Tuple 3, violating key uniqueness.
Therefore, $X$ should NOT be equal to $19$ .

Final answer

Common trap

Looking at StudentAge across other student names (like Swati or Ganesh) instead of the same student name Shankar.

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