Suppose $f:[0, 1]\rightarrow \mathbb{R}$ is given by
$$f(x)= \begin{cases} x &\text{if}\; x \in \mathbb{Q} \\ 0 &\text{if}\; x \notin \mathbb{Q} \end{cases}$$
Find lower and upper Riemann integral on $[0, 1]$ interval for this function.
I know, looks like easy question. But I'm not sure for upper Riemann integral. Can you help me, please?