Question 9493 – GATE 2004
November 12, 2023Question 9901 – Database-Management-System
November 12, 2023Question 10005 – Database-Management-System
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?
Correct Answer: D
Question 19 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.
σC1(σC1(R1)) → σC2(σC2(R1))
σC1(σA1(R1)) → σA1(σC1(R1))
σC1(R1 ∪ R2) → σC1(R1) ∪ σC1
πA1(σC1(R1)) → σC1(σA1(R1))
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