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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