Database Design and Management Question (Unrelated to Normalization this time)

[Mystic Muse]

Hey, working on some homework, and am just a little bit confused about this one question.

16. What type of entity whose logical existence in a database depends on the existence of another entity?

I've successfully answered almost all of the other questions so far. This is not a timed test, and it's open book, so no worries about how quickly it's answered or if somebody helps me, so long as it's in by the 4th.

I'm just not entirely sure what it is they want me to figure out.

[ChristianSt]

Huh. Is this even a correct question? :smallconfused:
Because to me it seems like it would miss some words or something. At least I can't really say what is being asked.

That being said: From what I understand from that, I'm pretty sure that the answer is (or is related to) weak entities (http://en.wikipedia.org/wiki/Weak_entity).

A normal entity (or strong entity) must be uniquely identified somehow (attribute(s) with this property can function as primary key later).

A weak entity doesn't have that. On its own it cannot be identified. It is only uniquely identifiable by a identifying relationship to another entity. So later the primary key consist of some information of the weak entity, known as the partial key, and the foreign key to the entity of the identifying relationship.


Whether an entity is weak or strong depends heavily on how you model things. A "typical" example of strong/weak entities would be the following:
Strong entity: A building with the primary key "address".
Weak entity: A room with a partial key "room number".

If you later see a "room number" you cannot say which room is being talked about. Only with a corresponding address you can identify the room.

[Mystic Muse]

Huh. Is this even a correct question? :smallconfused:
Because to me it seems like it would miss some words or something. At least I can't really say what is being asked.


Possibly not, though your answer seems correct. Thank you for the help. :smallsmile: