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 = πA(σC<10(R)) and Q2 = σC<10(πA(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!