ERD (Entity-Relation Diagram)
My preferred data model diagramming style is to use only Entity “boxes” connected via Crow’s Foot Notation.
However, in my experience, a Normalised model mainly finishes up with only exactly-one to zero-or-more connections:
In other words, in a Normalised model, the important thing to know is which is the “many” end. Knowing that, we know the “many” is also optional, and the other end is always singularly mandatory.
Hence, I tend to simplify the diagram to simply show only that “many” end:
On occasion, the “many” end might be trivially zero-or-one, in which case I will replace the “crow’s foot” with the conventional ring + vertical bar, so that the optional end is still clear.