Entity Relationship Diagram - Relationship Strength?

Question

Hi,

I am trying to figure out under what circumstances I should use weak (non-identifying) relationships (where the primary key of the related entity does not contain a primary key component of the parent entity), verses when I should use strong (identifying) relationships (primary key of the related entity contains a primary key component of the parent entity).

For example, when designing an Entity Relationship Diagram , if I have two entities, (e.g. book and purchaser), how do I know when to choose the solid Crows Foot or the dashed Crows Foot to connect the two entities?

Any assistance will be appreciated. Thanks in advance.

Answer 1

The rule is "Independent Existence"

A book exists whether it is purchased or not.

A purchaser exists whether they purchase a book or not.

However, a sales receipt for the purchase of a book cannot exist without purchaser and book.