GATE | GATE 2017 MOCK II | Question 55

A student relation with 4 attributes is defines as R(ABCD). Which of the following query will execute without error if all attributes are numeric.

B) σA>20(∏B,C)
(A) Only A
(B) Only B
(C) Both
(D) None

Answer: (A)

Explanation: Query in A is correct. It will first select required rows and then project required columns. But query in B is not correct as after projecting B and C columns, it can’t have row selection based on condition on attribute A. So correct option is 1.

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