ERD notations differ

Entity-Relationship diagrams (ERD) emerged in the 1970's from work by Dr. Peter Chen and others. They were looking for means to simplify the representation of large and complex data storage concepts.

Since ERD relationships are between entities, the ERD is not capable of expressing intra-entity, inter- attribute relationships - i.e., within one entity. This can be done:

• By certain extensions to the ERD in some methods
• With unmodeled text rules and constrains
• Using Object Role Modeling for the conceptual model