Weak entity types

a)
Weak entity type cannot be uniquely identified by its own attributes alone and thus needs another entity to be uniquely identified. So in relational model, every relation which has primary key made of foreign key and perhaps some other attribute, is weak entity type?

Ok, but I could instead of creating a foreign key create another attribute which could uniquely identify rows in a table. By definition the relation would no longer be weak entity type --> there has to be more to this --> perhaps it’s more of a subjective thing?! Meaning two tables can both have compound primary key, but one table could be considered weak and other strong entity type, based on how the person creating the two tables would perceive the world?! Can you show me an example?

If so, aren’t there some regulations that would in more objective way define when an entity is weak and when strong ( assuming entity has compound primary key in both cases )

b)
Looking at few E-R diagrams I noticed that attributes being drawn for particular entity type ( entity type is drawn as rectangle ) often don’t include an attribute acting as foreign key  I’d understand if this entity type was weak entity type and thus would include foreign key attribute only when E-R model was converted into, say, relational model, but in the examples I saw the entity type wasn’t represented as a weak type. So when do we also draw foreign key attributes and when not?!

thank you Received on Sun Aug 12 2007 - 00:55:29 CEST