4.4

Entities in Database Design

Strong Entities

• A strong entity is defined as an entity that can exist independently of other entity types.

• It has its own identifier, known as the primary key, which differentiates it from other entities.

• When implemented as a table, a strong entity must contain one or more identifying attributes.

• One of these identifying attributes is typically the primary key of the table.

• Identifying relationships are crucial in establishing connections between entities, where one entity can be linked to another based on these attributes.

Weak Entities

• A weak entity is one that cannot be uniquely identified by its own attributes alone; it relies on a strong entity or another weak entity for identification.

• Weak entities are usually identified by a strong entity, which acts as a parent, ensuring its uniqueness.

• Alternatively, a weak entity can also be identified by another weak entity in some cases, creating a chain of dependency.

• Relationships can vary:

  • One strong entity can identify one weak entity.

  • One weak entity may be identified by multiple strong entities or other weak entities.

• Database designers prioritize identifying relationships before documenting weak entities to ensure structure and integrity in the database.