1

The following relation schema: R(A,C), S(A,D), T(X,Y). Primary key attributes bold

Would like to know why the following two are not equivalent queries

Q1 = πAC<10(R)) and Q2 = σC<10A(R))

The answer: Q2 Invalid expression as the selection condition refers to a non-existent attribute. I'm not sure what the answer means. Q1 and Q2 looks similar but Q2 is invalid.

Could anyone provide some values and examples to explain? Thanks!

  • You quote an explanation. Well, what is the definition of selection? So, what is a selection condition? So what is an attribute referred to by a selection condition? What are the ones here? What does each subexpression here mean? Please google 'stackexchange homework' & show some effort & show your work until you are stuck. Also read the downvote arrow mouseover text. – philipxy Mar 25 '18 at 2:31

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