Inside every interval, no matter how small, there exist both rational and irrational numbers. If, while doing your "Riemann sums", you evaluated f(x) only at irrational numbers you function would look exactly like y= x. What is the integral of that from 0 to 1? If, while doing your "Riemann sums", you evaluated f(x) only at rational numbers you function would look exactly like y= -x. What is the integral of that from 0 to 1?