识别关系与构成关系？

Question

这两个有什么不同？由于两个关系都由一组对象组成，所以A不能独立存在，而另一组对象则说B存在对于A组中对象的存在是必要的。

这两个相同或者是我错过的一些基本差异吗？

Answer 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.