Still Confused About Identifying vs. Non-Identifyi

Question

So, I've been reading up on identifying vs. non-identifying relationships in my database design, and a number of the answers on SO seem contradicting to me. Here are the two questions I am looking at:

1. What's the Difference Between Identifying and Non-Identifying Relationships
2. Trouble Deciding on Identifying or Non-Identifying Relationship

Looking at the top answers from each question, I appear to get two different ideas of what an identifying relationship is.

The first question's response says that an identifying relationship "describes a situation in which the existence of a row in the child table depends on a row in the parent table." An example of this that is given is, "An author can write many books (1-to-n relationship), but a book cannot exist without an author." That makes sense to me.

However, when I read the response to question two, I get confused as it says, "if a child identifies its parent, it is an identifying relationship." The answer then goes on to give examples such as Social Security Number (is identifying of a Person), but an address is not (because many people can live at an address). To me, this sounds more like a case of the decision between primary key and non-primary key.

My own gut feeling (and additional research on other sites) points to the first question and its response being correct. However, I wanted to verify before I continued forward as I don't want to learn something wrong as I am working to understand database design. Thanks in advance.

Answer 1

part of the issue here is the confusion of terminology. identifying relationships are useful for avoiding long join paths.

The best definition i have seen is "an identifying relationship includes the PK as of the parent in the the child PK. In other words the PK of the child includes the FK to the parent as well as the "actual" PK of the child.

Answer 2

I believe only difference between an identifying and non identifying relationship is about Nullability of the foreign key. If a FK cannot be NULL, the relationship it represents is identifying (child cannot exist without parent) else it is non identifying.