GATE CS 2014 Set 2 — Question 32
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Databases → Relational Model → Keys (Super, Candidate, Foreign)
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Question
Given an instance of the STUDENTS relation as shown below:
For (StudentName, StudentAge) to be a key for this instance, the value X should NOT be equal to ___________.
| 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 |
Correct answer
19 to 19
Solution
A candidate key (or simply a key) in a database relation must uniquely identify each tuple (row). This means that no two tuples can have the same values for the key attributes.We are given that the pair is a key. Let's examine the values of this pair for each row in the provided instance:
1.(Shankar, X)
2.(Swati, 19)
3.(Shankar, 19)
4.(Swati, 18)
5.(Ganesh, 19)
For the key constraint to hold, all these pairs must be distinct. Comparing the first pair (Shankar, X) with the third pair (Shankar, 19):- The StudentName is 'Shankar' in both.
- If were equal to 19, the first pair would be (Shankar, 19), which is identical to the third pair. This would violate the uniqueness property of the key.
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