绘制ER图时局部性和可选性有什么区别 [英] What is the difference of partiality and optionality when drawing ER diagrams
问题描述
部分关系用单行显示，总关系用双行显示。在某些ER图中，可选关系和强制关系分别用虚线和单线表示。
Partial relationships are shown by single lines total relationships are shown by double lines. And in some ER diagrams optional and mandatory relationship is shown by a dotted line and a single line respectively.
这些概念之间的定义有何确切区别？如果可能，请使用示例。
What is the exact difference of definitions between these concepts? Please use an example if possible.
推荐答案
请考虑以下示例：
A 患者或实验室可以独立于任何关系而存在。某些患者可能购买 医学，而某些实验室可能生产 医学，因此这些实体集部分参与了它们的关系
A Patient or a Laboratory may exist independently of any relationship. Some Patients may Purchase Medicine, and some Laboratories may Produce Medicine, hence those entity sets participate partially in their relationships.
医学不存在，除非由生产 实验室，因此医学完全参与了 Produced 的关系。 医学可以存在而无需购买，因此医学部分参与购买关系。
Medicine cannot exist without being Produced by a Laboratory, thus Medicine participate totally in the Produced relationship. Medicine can exist without being Purchased hence Medicine participate partially in the Purchase relationship.
A 患者可能购买一些药物而没有处方，因此处方对于购买关系是可选的。相反，没有患者和医学购买是不存在的>，因此这些角色对于关系是必不可少的。在生产的关系中，这两个角色都是强制性的。
A Patient may Purchase some Medicine without a Prescription, thus a Prescription is optional to the Buys relationship. In contrast, a Purchase cannot exist without a Patient and Medicine, hence those roles are mandatory to the relationship. In the Produced relationship, both roles are mandatory.
此处的基本区别是基数限制了关系的存在，而参与限制了实体的存在。如果关系集中的角色是强制性的，则意味着没有实体填充该角色就无法存在关系实例。当一个实体集完全参与一个关系时，这意味着一个实体不存在就不会存在。
The basic difference here is that cardinality constrains the existence of relationships, while participation constrains the existence of entities. When a role in a relationship set is mandatory, it means a relationship instance can't exist without an entity filling that role. When an entity set participates totally in a relationship, it means an entity can't exist without being related.
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