When it comes to database design, a weak entity is also commonly referred to as a dependent entity. This term is used to emphasize the fact that a weak entity relies on another entity, known as its owner entity, for its existence. The owner entity is typically a strong or regular entity that has a primary key, whereas the weak entity does not have its own unique identifier.
To better understand the concept of a weak entity, let’s consider an example from the real world. Imagine you are designing a database for a library. One of the entities in this database could be a “Book” entity, which represents individual books in the library. Now, each book may have multiple copies, and these copies are essentially the weak entities in this scenario. They depend on the existence of the book itself, as they cannot exist without it.
In this example, the “Book” entity is the owner entity, and the weak entity is the “Copy” entity. The primary key of the “Book” entity, such as the ISBN, is used as a foreign key in the “Copy” entity to establish the relationship between them. This way, each copy can be uniquely identified by its associated book.
It is important to note that there are two types of weak entities: associative entities and subtype entities. Associative entities are used to represent relationships between two or more entities. For instance, in our library example, we could have an entity called “Borrowing” that represents the relationship between a “Copy” entity and a “Member” entity, indicating which member borrowed which copy and when.
On the other hand, subtype entities are used to represent specialized types of an entity. For example, in a database for a university, the “Person” entity could have subtypes like “Student” and “Faculty,” each having their own specific attributes. The discriminator, often represented by a column in the parent entity, determines the subtype of the entity.
The term “dependent entity” is another name for a weak entity. It signifies the reliance of the weak entity on an owner entity for its existence. Additionally, it is worth remembering that weak entities can be further categorized into associative entities and subtype entities, depending on their purpose in the database design.