CODEWITHSUNDEEP
databaserdbmsentity-relationship
SHUBHAM TANDAN
SHUBHAM TANDAN

Reputation: 71

Weak entity dbms

What i know about weak entity is that they cannot exist without strong entity:-

i.e there must be a strong entity from which key(foreign key) has to be taken by weak entity.

example : a HOTEL(strong entity) and ROOM(weak entity), HOTEL is strong entity because they can have their existence by their own (independently) while ROOM is weak entity as they always need HOTEL without it (HOTEL) room cannot exists.

But while reading book i came across a statement which states that "However not every existence dependency results in weak entity type ".For example a DRIVER_LICENCE entity cannot exist unless it is related to a PERSON entity ,even though it has its own key (licence_number) and hence it is not a weak entity .

My question is how an entity depending upon strong entity cannot be a weak entity?

Upvotes: 1

Views: 962

Answers (1)

reaanb
reaanb

Reputation: 10065

Weak entities aren't defined as being dependent on another for existence, but as being dependent on another for identity.

Quoting from Peter Chen's 1976 paper "The Entity-Relationship Model - Toward a Unified View of Data":

...we have two forms of entity relations. If relationships are used for identifying the entities, we shall call it a weak entity relation. If relationships are not used for identifying the entities, we shall call it a regular entity relation. Similarly, we also have two forms of relationship relations. If all entities in the relationship are identified by their own attribute values, we shall call it a regular relationship relation. If some entities in the relationship are identified by other relationships, we shall call it a weak relationship relation. For example, any relationships between DEPENDENT entities and other entities will result in weak relationship relations, since a DEPENDENT entity is identified by its name and its relationship with an EMPLOYEE entity. The distinction between regular (entity/relationship) relations and weak (entity/relationship) relations will be useful in maintaining data integrity.

Thus, an entity that is identified by its own attributes is called a regular entity, regardless of whether it can exist independently or only in relation to another entity.

An entity that is identified by its relationship with another entity is necessarily dependent on it for existence, but that argument doesn't work the other way around. If it rains, the ground is wet, but just because the ground is wet doesn't mean it rained.

Upvotes: 3

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