The Definition of Entity -ERD

My understanding of the definition of Entity, relating to Entity-Relationship Diagram and relational data management.

Before I explain my idea, I want to include a picture i drew. Please ignore the quality as it is just to use help express and understand the definition of Entity in ERD.

Entity, ERD, Entity-Relationship Diagram

When we decide if we need to consider something as an entity and include it a source table, we need to think about

Can existing tables already include all the attributes that we need for this entity?

If the answer is yes, then we should not consider this as an entity. In stead, we can see this "something" as a composite view of Entity A and Entity B by joining.

If the answer is no, then we should consider this as an entity and include it in the source table.

Also, I want to list Copilot's explanation here:

When deciding whether to model something as an entity in an ERD (and include it as a source table), ask:

🔹 “Can all the attributes of this concept be fully and correctly derived from existing tables and relationships?”

• If YES →

  • It’s a composite view.

  • You can represent it by joining existing entities.

  • No need to create a new table.

• If NO →

• It’s a distinct entity.

• It has attributes that belong to it alone.

• You must include it as a source table in your schema.

🧠 Expressive Metaphor

• If the concept is just a lens, it’s a view.

• If it’s a ledger, it’s an entity.

• If it adds new meaning, it deserves its own vault.