EntityRelationshipModellierung

EntityRelation, commonly referred to as the Entity-Relationship model (ER model), is a conceptual data model used in database design to describe data and its relationships. It represents real-world objects as entities, each with attributes, and the relationships that connect those entities. Entities denote distinct objects with a unique identifier (primary key). Attributes describe properties of entities; they can be simple, composite, multi-valued, or derived.

Relationships link entities and specify how instances relate. Relationships have cardinality constraints (one-to-one, one-to-many, many-to-many) and participation constraints (total or partial). ER diagrams visually encode the model, often using Chen notation or other variants like Crow’s Foot.

History and purpose: Introduced by Peter Chen in 1976 to provide a high-level, database-agnostic view. The ER model serves as a design tool before implementing a relational schema. Mapping from ER to relational schema is a common step: each entity becomes a table, attributes become columns, and relationships are implemented via foreign keys or join tables for many-to-many.

Extensions and notation: Extended ER (EER) adds features such as specialization/generalization (inheritance), categories, and additional constraints. Notation variants exist to emphasize different aspects; Chen notation uses diamonds for relationships, rectangles for entities, and ovals for attributes, but other notations exist.

Relation to other models and limitations: ER is a planning stage tool for relational databases. It complements UML class diagrams and relational schemas. It can be less suited for some NoSQL or schema-less systems, and large domains can become complex to maintain.