GATE 2004
November 12, 2023Database-Management-System
November 12, 2023Database-Management-System
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Question 30
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Which of the following query transformations (i.e. replacing the l.h.s. expression by the r.h.s. expression) is incorrect? R1 and R2 are relations, C1, C2 are selection conditions and A1, A2 are attributes of R1?
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σC1(σC1(R1)) → σC2(σC2(R1))
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σC1(σA1(R1)) → σA1(σC1(R1))
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σC1(R1 ∪ R2) → σC1(R1) ∪ σC1
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πA1(σC1(R1)) → σC1(σA1(R1))
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Question 30 Explanation:
If the selection condition is on attribute A2, then we cannot replace it by RHS as there will not be any attribute A2 due to projection of A1 only.
Correct Answer: D
Question 30 Explanation:
If the selection condition is on attribute A2, then we cannot replace it by RHS as there will not be any attribute A2 due to projection of A1 only.