A weak entity, by definition, does not have a primary key. In a relational database model, an entity set is a collection of similar entities that can be uniquely identified by their attributes. A primary key is a specific attribute or combination of attributes that uniquely identifies each record in an entity set.
However, in the case of a weak entity set, it does not have enough attributes to form a primary key on its own. A weak entity is dependent on another entity, known as its owner entity, to provide a partial key known as a discriminator. The discriminator, along with the weak entity’s own attributes, forms a combined primary key.
To better understand this concept, let’s consider an example. Imagine we have a database for a library, and we want to model the relationship between books and their copies. The book entity would be the owner entity, and the copy entity would be the weak entity.
The book entity would have attributes such as book_id, title, author, and publication date. The copy entity, on the other hand, may have attributes like copy_id, location, and condition. However, the copy_id attribute alone cannot uniquely identify a copy since there can be multiple copies of the same book with the same copy_id.
In this case, the primary key for the copy entity would be a combination of the discriminator attribute (book_id) from the owner entity (book) and the copy_id attribute from the weak entity (copy). This combined primary key ensures that each copy of a book is uniquely identified within the database.
It is important to note that the weak entity’s primary key is only unique within the context of its owner entity. In other words, the combination of the discriminator and weak entity’s attributes forms a primary key for the weak entity set, but it is not unique across the entire database.
A weak entity does not have a primary key on its own. It relies on its owner entity to provide a partial key, known as a discriminator, which, when combined with the weak entity’s attributes, forms a combined primary key. This combined primary key ensures the uniqueness of each record within the weak entity set, but it is not unique across the entire database.