isro cs question paper solution 2018 Q5 Set-A

Question 5

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Consider the following table in a relational database

Last Name	Rank	Room	Shift
Smith	Manager	234	Morning
Jones	Custodian	33	Afternoon
Smith	Custodian	33	Evening
Doe	Clerical	222	Morning

According to the data shown in the table, which of the following could be a candidate key of the table?

(a) { Last Name}

(b) { Room }

(c) { Shift }

(d) { Room, Shift }

Correct answer:  (d) { Room, Shift }

Explanation: 

Candidate key is the attribute or set of attribute which uniquely identifies the tuple.  

Smith not uniquely identifies the tuple: as Smith-> manager (Rank) and Smith-> Custodian (Rank).

Similarly, Room gives different Shift value for key 33 (Room).

Similarly, Shift gives different Room value for key Morning (Shift).

So, after combining the { Room, Shift } as a candidate key to uniquely identify the tuples.