Which of the following query transformations (i.e., replacing the l.h.s.…

GATE · 1998 · CS · Question 2 subpartsModified — slightly modified from the official paper; see the solution

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.

A.
σ C₁ (σ C₂ R₁) → σ C₂ (σ C₁ (R₁))
B.
σ C₁ (π A₁ R₁) → π A₁ (σ C₁ (R₁))
C.
σ C₁ (R₁ ∪ R₂) → σ C₁ (R₁) ∪ σ C₁ (R₂)
D.
π A₁ (σ C₁ (R₁)) → σ C₁ (π A₁ (R₁))

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Correct answer: D

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