UGC NET CSE | June 2016 | Part 2 | Question: 20

Question

Consider the following database table having $\text{A, B, C}$ and $\text{D}$ as its four attributes and four possible candidate keys $\text{(I, II, III and IV)}$ for this table :

$\begin{array}{|l|l|l|l|} \hline \text{A} & \text{B} & \text{C} & \text{D} \\ \hline \text{$a_1$} & \text{$b_1$} & \text{$c_1$} & \text{$d_1$}  \\ \hline \text{$a_2$} & \text{$b_3$} & \text{$c_3$} & \text{$d_1$}  \\ \hline  \text{$a_1$} & \text{$b_2$} & \text{$c_1$} & \text{$d_2$}  \\ \hline \end{array}$

$\begin{array}{} \text{I : {B}} & \text{II : {B, C}} & \text{III : {A, D}} & \text{IV : {C, D}} \end{array}$

If different symbols stand for different values in the table $(\text{e.g.,}\; d_1$ is definitely not equal to $d_2),$ then which of the above could not be the candidate key for the database table?

• A. $\text{I}$ and $\text{III}$ only
• B. $\text{III}$ and $\text{IV}$ only
• C. $\text{II}$ only
• D. $\text{I}$ only

Answer 1

Ans is C

1:(B)

2: ((B,C)

3: (A,D)

4: (C,D)

1 is definetly canidate key

2 BC canot be candidate key because it is Superset of candidate key(B) thats why it is superkey

3 AD is candidate key it is minimal superkey

4 CD is also candidate key it is also minimal superkey

Comments

Can you provide its explanation

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Super key is any set of attributes that uniquely determine

Candidate key / key is minimal super key wherein if we remove some attriubtes also it is still a super key

so here {B,C} is super key, but its not minimal because if we remove C, B alone can also uniquely detemine the tuples

So B is candidate key, whereas {B,C} is not

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@subhampandey your explanation is right but at the top you made a mistake.Ans should be 2.(B,C)

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Yess

Answer 2

Candidate Key Is a minimal Super Key Then If B is a Candidate Key ,  How possible That BC is Also A Candidate  Key

 

 

option B is Not Possible As a candidate key

So, correct option is B