19 Sep (Melanie)An entity is a real-world object, such as a customer, a car, a class, or a student.
(Melanie)An entity is a real-world object, such as a customer, a car, a class, or a student. (Coronel.) A relationship is an association between entities, identified with a verb: a customer buys a car and a student takes a class. Relationships between entities operate in both directions – a car is bought by a customer and a class is taken by many students. In a one-to-many relationship, the “one” is normally considered a parent entity, and the entity on the “many” side is usually considered the child or related entity.
The strength of a relationship is based on how the primary keys of the related entities are defined. (Coronel.) Relationships are established by having the primary key of the parent entity included among the attributes of the child entity as a foreign key. In a weak, non-identifying relationship, the primary key of the child entity does not contain a primary key component of the parent entity. On the other hand, in a strong, identifying relationship, the primary key of the related entity contains a primary key component of the parent entity. The strength of the relationship depends on whether the primary key of the child entity includes and thus identifies the primary key of the parent entity.
In addition to relationships, entities can be strong or weak as well. A weak entity must meet two conditions: 1) the entity cannot exist without the parent entity and, 2) the entity has a primary key that is partially or totally derived from the parent entity in the relationship. (Coronel.) A loan cannot exist without a borrower and a dependent for a company insurance policy cannot exist without an employee. Because a weak entity, by definition, must include a primary key component of the parent entity as part of its own primary key, a weak entity can never be in a weak relationship.
Reference:
Coronel, C., & Morris, S. (2019). Database systems: Design, implementation, and management (13th e). Cengage Learning.
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