弱实体数据库
Weak entity dbms
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密钥（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 "However not every existence dependency results in weak entity type "。例如，一个 DRIVER_LICENCE 实体不能存在，除非它与一个 PERSON 实体相关，即使它有自己的密钥(licence_number) 因此它不是弱实体。
我的问题是一个依赖于强实体的实体如何不能成为一个弱实体？
弱实体的存在不依赖于另一个实体，而是定义为身份依赖于另一个实体。
引自 Peter Chen 1976 年的论文"The Entity-Relationship Model - Toward a Unified View of Data"：
...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。
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密钥（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 "However not every existence dependency results in weak entity type "。例如，一个 DRIVER_LICENCE 实体不能存在，除非它与一个 PERSON 实体相关，即使它有自己的密钥(licence_number) 因此它不是弱实体。
我的问题是一个依赖于强实体的实体如何不能成为一个弱实体？
弱实体的存在不依赖于另一个实体，而是定义为身份依赖于另一个实体。
引自 Peter Chen 1976 年的论文"The Entity-Relationship Model - Toward a Unified View of Data"：
...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。