ER 设计中的弱实体
我有一个弱实体，它有一个 id 作为其主键，并且还有另一个属性 (author_id)，它必须引用另一个实体的 (Author)代码>'s) 主键。
所以我的问题是：弱实体可以有复合键（id，author_id）吗？
I have a weak entity and it has an id as its primary key and also has another attribute (author_id) which has to refer to another entity's (Author's) primary key.
So my question is this: can a weak entity have a composite key (id, author_id)?
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如果 Id 是键，那么 (Id, Authorid) 不可能是同一个表中的键。没有一个键是另一个键的子集，因为根据定义，键应该是不可约唯一的。
你的问题的答案是肯定的。弱实体可能具有复合密钥。
If Id is a key then (Id, Authorid) can't possibly be a key in the same table. No key is ever a subset of another because keys by definition should be irreducibly unique.
The answer to your question is yes. A weak entity may have a composite key.
弱实体是一种具有主键的实体，该主键至少包含一个外键，但不仅仅包含外键。仅使用一种属性这是不可能的。因此，
id，那么您就没有弱实体，而是强实体；id、whatever，并且id本身已经是唯一的，那么id本身应该成为你的主钥匙；id、whatever，并且id和whatever本身都不是唯一的，但是 < code>id,whatever在一起是唯一的，那么它们是合法的主键，并且id、whatever一起作为外键，那么你就没有弱实体，而是子实体（外键是IS-A关系）；id和whatever都是外键，则您没有弱实体，而是关系；id和whatever之一是外键，而另一个不是，则您的实体是弱实体。所有这些情况都可能发生。这取决于您想要建模的内容以及您拥有哪些相关信息。
A weak entity is an entity with a primary key that contains at least one of its foreign keys, but that doesn't contain only foreign keys. This is not possible with only one attribute. So
id, you do not have a weak entity, but a strong entity;id,whatever, andidis already unique by itself, thenidby itself should be your primary key;id,whatever, and neitheridnorwhateverare not unique by themselves, butid,whatevertogether are unique, then they are a legitimate primary key, andid,whateverare a foreign key together, you do not have a weak entity, but a subentity (the foreign key is an IS-A relationship);idandwhateverare both foreign keys, you do not have a weak entity, but a relationship;idandwhateveris a foreign key and the other is not, you have a weak entity.All of these cases may occur. It depends on what you want to model and which information you have about it.