Correct entity relationship diagram representation of a normalized table with a NOT NULL foreign key

## Intro Hi everyone, this is mainly a classification/theoretical question on the topic of **inheritance** and **normalization** techniques in database design and their appropriate representation in entity relational diagrams. In a practical implementation it doesn't present a challenge as you could just set the attribute in question as **IS NOT NULL** and be done. However for a graphical representation I am a little confused on how to do it correctly. Here is a github gist link with a interactive **mermaid** diagram representing a hypothetical scenario on a particular example: [gist](https://gist.github.com/papshmeare/dfe4b93e7c829b486f382e78e30c6318) ## Problem The important part is in this section:  Assume we have an entity called PRODUCT which might have different types of attributes depending on the product in question(eg. Physical product/Digital product). For that reason we introduce two "subtables" whose PKs refer to a PRODUCTs FK attribute **product_type_id**. It is clear that a PRODUCT can only have a singular **product_type_id** but because it can be either **PhysicalProduct** or **DigitalProduct** what kind of relationship do these two "subtables" have with PRODUCT? So far I deduced that it must be **one to (zero or one)** as presented in the graph. That's where the problem(perhaps non-existent) lies. If we have two **one to (zero or one)** relationships to a mandatory **IS NOT NULL** attribute doesn't it infer a possibility of two **one to zero** relationships visually or is it something not clicking in my head here and that is how it is supposed to be in this kind of scenario?