Below is a sentence from a grammar book.

I have nothing of which to complain.

The book says the phrase "of which" is used as part of a relative pronoun clause. Suddenly this came confusing to me as I felt I had heard other many similar expressions like that in its shape but with different interpretation of meaning. For example I made up a sentence as follows.

I have to make a decision of which to choose.

Is also the second sentence right above part of a relative pronoun clause? Or is it totally different grammatically?

Thank you very much.