识别关系与构图关系？ [英] Identifying relationships vs composition relationships?

问题描述

两者有何不同？由于这两种关系都由一组不能独立存在的对象A组成，而另一组对象B的存在对于组A中对象的存在是必需的。B

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.

那么两者是相同的，还是我遗漏的根本区别？

So are the two same or are their some fundamental differences that I am missing out on?

推荐答案

构成关系在ER中不是正式的关系类型。如果我认为它是二元关系（a，b），它是左加总的（每个a与某些b相关），其中 a-> b （a仅与一个b相关），则识别关系将是一种特殊的组成关系。标识关系的格式为（（x，b），b），其中（x，b）-> b 。然后，非标识组成关系将包括通过其自身属性标识组件集的情况。

"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.

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