Explain the concept of a strong entity in Entity Relationship modelling.

A strong entity in Entity Relationship modelling is an entity that can exist independently without needing to rely on another entity.

In the realm of Entity Relationship (ER) modelling, a strong entity is a type of entity that doesn't need to depend on another entity to be uniquely identifiable. It has a primary key of its own, which is a unique identifier that distinguishes each instance of an entity. This means that each instance of a strong entity is defined by its own attributes, rather than by its relationship with other entities.

For example, consider a university database system. In this system, 'Student' could be a strong entity, as each student can be uniquely identified by their student ID, which is independent of any other entities. The student ID serves as the primary key for the 'Student' entity. This means that even without any relationships to other entities, such as 'Course' or 'Lecturer', a 'Student' entity can still exist in the database.

Strong entities are the backbone of an ER model, as they form the primary structure upon which other entities and relationships are built. They are typically the first entities to be defined in the modelling process, as they provide the fundamental units of data that the system will manage.

In contrast to strong entities, there are also weak entities. Weak entities are entities that cannot exist without being associated with a strong entity, as they do not have a primary key of their own. Instead, they rely on a foreign key, which is a reference to the primary key of the strong entity they are associated with. For example, in the university database system, 'Course Registration' could be a weak entity, as it relies on the 'Student' entity (a strong entity) to exist.

In summary, a strong entity in ER modelling is an independent entity that has its own unique identifier, or primary key. It forms the fundamental structure of the ER model and can exist without relying on other entities.