First of all, the complete works is a relative concept. A complete set in one problem is not necessarily a complete set in another problem, that is to say, a complete set is to find a "largest" set in a specific problem, and all other sets are subsets of it, so when studying a problem, it can only be studied within the prescribed scope.

Secondly, a complete set can be an empty set. General topics do not define an empty set as a complete set, which is determined by the topic itself. For example, a function problem, the complete set is empty, that is, the independent variable does not take any number, how to solve it? In a topic, the complete set is defined as an empty set, and the topic is meaningless, so the complete set cannot be an empty set.