Re: Are circular relationships a bad thing in ER-modeling?

"x" <x-false_at_yahoo.com> wrote
> Show is an ENTITY or a RELATIONSHIP ? :-)

> What you usually use to relate two entities in ER-modeling ?

Sometimes, things that start out as RELATIONSHIPS eventually become recognized as ENTITIES in and of themselves. An example:

A "confirmation" in an airline reservation system. Initially this can be modeled as a ternary relationship between a flight, a date, and a passenger. But, as data like "confirmation status", and "confirmation number" get added, it is sometimes useful to model a confirmation as an entity in its own right. This is sometimes called "reifying".

.................

You generally don't "use" anything to implement the relationships in ER-modeling. You can represent relationships in an ER diagram by the lines that connect the boxes. But how you implement them depends on your implementation environment.
And a many-to-many relationship, in ER, is just that. No fancy tricks are needed, because ER doesn't model an implementation.

If you are implementing in a "navigational" (hierarchical or network) environment, you use pointers to express relationships.

If you are implementing in a "data centered" (relational or SQL) environment, you use foreign keys to express relationaships.

If you are implementing in an Object oriented environment, I don't know what you use.

HTH, Received on Mon May 03 2004 - 16:50:43 CEST