DBMS

Question

The correct answer is a and d. I just wanted to know that why not b and c. It is possible that it may be speed and color is also a candidate key. If you have any clarification on this let me know!!!

Comments

the question already mentions all the superkeys which are in the relation.

candidate keys are superkeys too.

B,C option is not mentioned in the list of superkeys which means they cannot be candidate keys

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i think ans only A because here given super keys are not All super keys.

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As mentioned in the question that superkeys are:- {VID},{VID,MODEL},{SPEED,COLOR},{SPEED,COLOR,MODEL}.

According to concept of keys:- “Every candidate key is super key but all super keys are not candidate key.”

If {VID,Model} is candidate key then {VID} cannot be superkey. But if {VID} is candidate key then {VID,MODEL} can superkey.

Likewise for other two it is same.

So, B and C option is incorrect.

Answer 1

I just wanted to know that why not b and c. It is possible that it may be speed and color is also a candidate key. 

As mentioned in the question, {speed, color} combo is superkey alone speed and color is not a superkey. So answer is A & D
 

Answer 2

all possible keys are called as super keys 
and minimal super keys ( that cant be more divided ) is only candidate key 
option A is correct but question is asking for all possible keys so from option 
only A and D is correct