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.
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.