Question

For each of the following pairs of expressions, give instances of relations that show the expressions are not equivalent.
a. ΠA(R − S) and Π A (R) – ΠA (S)
b. σB < 4(AGmax(B)(R)) and AGmax(B)(σB<4(R))
c. In the preceding expressions, if both occurrences of max were replaced by min would the expressions be equivalent?
d. (R Θ S) Θ T and R Θ (S Θ T)
In other words, the natural left outer join is not associative.
e. σθ (E1 Θ E2) and E1 Θ σθ (E2), where θ uses only attributes from E2


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  • CreatedJanuary 29, 2011
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