实体关系-弱者是否可以像“一个”一样参与“一对多”关系 [英] Entity Relationships - Can a weak entity take part in a 'one to many' relationship as the 'one'

问题描述

使用以下实体关系结构，我正在努力弄清贷款实体和ITEM实体之间的关系是否有效？

With the following entity relationship structure I'm struggling to figure out if the relationship between the LOAN and ITEM entities is valid?

贷款弱的实体使用a loan_dateLeant的部分键以及来自CUSTOMER和ITEM的主键，以形成LOAN的主键。
但是，贷款与ITEM之间存在一对多关系，因为贷款可以包含多个项目。
但是可以肯定的是，这意味着如果借出了多个项目，则贷款记录的主键部分将具有两个item_id值？

The weak entity of LOAN uses a partial key of 'loan_dateLeant' and the primary keys from CUSTOMER and ITEM to form LOANs primary key. However LOAN has a 'one to many' relationship with ITEM as a loan can consist of more than one item. But surely this means that if more than one item is loaned, then the loan record will have two item_id values for part of its primary key?

推荐答案

是的，那是无效的-您不能使用一对多关系作为主键的定义部分。相反，您可能考虑添加唯一的 loan_id 作为主键的一部分，而不是贷款包含的项目；那么单笔贷款将由其客户和贷款ID（或客户，日期和贷款ID）定义。

You're right, that's not valid - you can't use a to-many relationship as a defining part of a primary key. What you might consider doing instead is adding a unique loan_id that you make part of your primary key, instead of the items the loan contains; then a single loan is defined by its customer and loan ID (or customer, date, and loan ID).

如果这不起作用，则使 date_leant 一个datetime字段，并提高其精度，直到您不可能（在系统的约束范围内）同时发生两笔借贷-一次借贷的可能性有多大客户在毫秒之内进行两次单独的贷款交易？

If that doesn't work, make date_leant a datetime field, and increase the precision on it until you can't possibly (within the constraints of your system) have two loans occur at the same time - how likely is it that a single customer transacts two separate loans within milliseconds of each other?

请注意，这并不妨碍LOAN实体以一个的身份参与该关系；这只是意味着您无法使用许多来定义弱实体。

Note this doesn't prevent the LOAN entity from taking part in the relationship as the "one"; it just means that you can't define the weak entity using the "many".

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