问题
how are the two different? Since both of the relationships consist of one set of objects say A which cannot exist independently and another set of objects say B whose existence is necessary for the existence of objects in the set A.
So are the two same or are their some fundamental differences that I am missing out on?
回答1:
"Composition relationship" isn't a formal relationship type in ER. If I take it to mean a binary relationship (a,b) which is left-total (every a is related to some b) and in which a->b (an a is related to only one b), then an identifying relationship would be a specific kind of composition relationship. An identifying relationship has the form ((x,b),b) in which (x,b)->b. Non-identifying composition relationships would then include cases in which the component set is identified by its own attributes.
来源：https://stackoverflow.com/questions/39171313/identifying-relationships-vs-composition-relationships