Union refers to a set formed by merging all the elements of two or more sets, and its essence is to merge the elements of multiple sets into one set. For example, the set A = {1, 2,3} and the set B = {2 2,3,4}, then the union of A and B is {1, 2,3,4}. A complete set refers to a general set, which contains all the elements involved in the research problem and is not limited to a specific set. Its nature contains all the elements, which is a basic concept. For example, when studying natural numbers, a complete set is the set n of natural numbers. As far as scope is concerned, the scope of union is all elements of two sets, while the scope of complete set is all elements, including those in union.