NOTE
Entity–Relationship Diagrams
Modeling entities, attributes, identifiers, and relationships for data-oriented system design.
This is a historical learning note and may contain outdated or incomplete understanding.
An ER diagram models persistent domain data through entities, their attributes, identifiers, and relationships. Cardinality such as one-to-one, one-to-many, and many-to-many describes how entity instances can be associated.
Use ER diagrams to reason about data ownership and schema structure before implementation. They are not a substitute for transaction, lifecycle, or service-boundary modeling: a relationship that exists in the database does not automatically imply that two concepts belong in one aggregate or service.