database - 弱实体数据库

Question

我对弱实体的了解是，如果没有强实体，它们就无法存在:-

即必须有一个强实体，弱实体必须从中获取 key (外键)。

示例:HOTEL(强实体)和 ROOM(弱实体)，HOTEL 是强实体，因为他们可以(独立)拥有自己的存在，而 ROOM 是弱实体，因为他们总是需要没有它的 HOTEL(HOTEL)房间不存在。

但是在阅读本书时，我遇到了一个声明，该声明指出“但是并非每个存在的依赖性都会导致弱实体类型”。例如，除非 DRIVER_LICENCE 实体与 PERSON 实体相关，否则它不能存在，即使它有自己的实体key (licence_number)，因此它不是弱实体。

我的问题是，一个依赖强实体的实体如何不能成为弱实体？

Answer 1

弱实体并不被定义为依赖于另一个实体存在，而是定义为依赖于另一个实体。

引自 Peter Chen 1976 年的论文“实体关系模型 - 统一数据 View ”:

...we have two forms of entity relations. If relationships are used for identifying the entities, we shall call it a weak entity relation. If relationships are not used for identifying the entities, we shall call it a regular entity relation. Similarly, we also have two forms of relationship relations. If all entities in the relationship are identified by their own attribute values, we shall call it a regular relationship relation. If some entities in the relationship are identified by other relationships, we shall call it a weak relationship relation. For example, any relationships between DEPENDENT entities and other entities will result in weak relationship relations, since a DEPENDENT entity is identified by its name and its relationship with an EMPLOYEE entity. The distinction between regular (entity/relationship) relations and weak (entity/relationship) relations will be useful in maintaining data integrity.

因此，由自身属性标识的实体称为常规实体，无论它是否可以独立存在或仅与另一个实体相关。

通过与另一个实体的关系来识别的实体必然依赖于它而存在，但该论点反之则不然。如果下雨，地面是湿的，但仅仅因为地面是湿的并不意味着下雨了。