database - 谁能从欧文模型中解释这些符号?
问题描述
解决方案
For the model you linked, while I can't explain why it's like that (I didn't design it), I can at least tell you what is being conveyed:
- A Dependent can have zero or more records in the Policy table, and each record in the Policy table is related to exactly one record in the Dependent table. The relationship between Dependent and Policy is identifying (both because the PK in Policy contains all the FK columns from Dependent, and also because the relationship line between them is a solid line)
- A Policy can be associated with at most 1 Employee, and an Employee can have at most 1 Policy. The relationship is non-identifying, both because the FK from Employee is below the line (not part of the PK) on Policy, and because the relationship line is dashed.
The model looks like it might be from ERwin, but I'm not so sure about the ERD. The ERD might be in Korth notation, but it still doesn't look quite right... As it is, the arrow from Dependent to Policy seems backwards. If it is denoting cardinality, it should point the other way. I am assuming this is meant to be a many-to-one relationship. In that case, I think it would be correct if you swap the line from Employee to Policy with the arrow from Dependent to Policy, with the arrow going from Policy and pointing to Employee.
Is this meant to be a locical/physical model split? That's my guess, and the logical modeler got their arrows mixed up for the relationship.
推荐阅读
- ios - iOS 14 上的 GoogleCast 框架崩溃
- java - 为什么Java将方法参数视为其局部变量?
- swift - Swift 通用函数不接受输入
- javascript - PHP没有访问Javascript cookie
- macos - 使用 NSAccessibility API/XCUITest 框架的 MacOS 上的 Firefox 自动化是否可行?
- python - ValueError:重复可能不包含负值
- javascript - 重复反应组件的最佳实践
- r - R 从下一个不相等的值中进行条件减法
- hibernate - 批量更新并返回 JPA/Hibernate 中失败更新的 Id
- swift - 在swiftui,ios14中触摸本地通知时重定向到工作表或演示文稿