GATE CSE 2014 Set 2 | Question: 22

Question

Given an instance of the STUDENTS relation as shown as below

$$\begin{array}{|c|c|c|c|c|} \hline \textbf {StudentID} & \textbf{StudentName} & \textbf{StudentEmail} & \textbf{StudentAge} &  \textbf{CPI} \\\hline \text{2345} & \text{Shankar} & \text{shankar@math} & \text{X} & \text{9.4}\\\hline\text{1287} & \text{Swati} & \text{swati@ee} & \text{19} & \text{9.5}\\\hline\text{7853} & \text{Shankar} & \text{shankar@cse} & \text{19} & \text{9.4} \\\hline \text{9876} & \text{Swati} & \text{swati@mech} & \text{18} & \text{9.3} \\\hline \text{8765} & \text{Ganesh} & \text{ganesh@civil} & \text{19} & \text{8.7} \\\hline \end{array}$$
For $(\text{StudentName, StudentAge})$ to be a key for this instance, the value $X$ should NOT be equal to______.

Comments

why not 18?

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 Pronomita Dey 1

value X should NOT be equal to 19 is correct answer.

if u say why not 18 ? then i ask u why not 17 ?

Answer 1

Should not eqaul to $19$.

Since if it is equal the same key will have two different values for "StudentEmail" which cannot be true by the definition of candidate/primary/super key.

Comments

The question is based on StudentName, StudentAge. But StudentEmail being referred in the answer. Am I missing something?

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$(StudentName, StudentAge)$ together form a key, so they functionally determine all other attributes in the set. So $(StudentName, StudentAge) → Email$ is a FD. This means that $(Shankar, 19) → shankar@math$ and $(Shankar, 19) → shankar@cse$ are not allowed.

Answer 2

There is already an entry with same name and age as 19. So the age of this entry must be something other than 19.

Answer 3

X Is NOt Equal 19

 

Because Primary Key  unique Key It Uniquely Identify All Tuples If It is  19  It has Same Value As Another Tuple So  Other Then 18 Id Preferble