ERmodellering

ERmodellering, also called entity-relationship modeling, is a method for structuring and visualizing data requirements for information systems by defining entities, attributes, and relationships. It aims to capture data semantics and constraints independently of a particular database technology, to support communication between stakeholders and to guide database design.

Core concepts: An entity represents a real-world object or concept with a distinct identity (e.g., Customer, Order). Attributes describe properties, such as CustomerID, Name, or OrderDate. Relationships indicate associations between entities, possibly with cardinality and participation constraints. Entities can be strong or weak; a weak entity relies on an identifying relationship with a parent entity. Keys: primary keys uniquely identify entity instances; foreign keys establish relationships in a relational implementation.

Notation and diagrams: ER diagrams depict entities as rectangles, attributes as ovals or lists, and relationships as diamonds (Chen notation) or as lines in other variants. Cardinality expresses how many instances participate: one-to-one, one-to-many, many-to-many; participation can be total or partial. Common notations include Chen and Crow's Foot; UML class diagrams can be used for related modeling tasks.

Process and mapping: The modeling process typically includes requirements gathering, identifying entities and attributes, defining relationships and constraints, and validating the model with stakeholders. The ER model is often transformed into a relational schema through mapping rules, after which normalization and physical database design are performed.

History and usage: Introduced by Peter Chen in 1976 to provide a high-level, declarative description of data. Today ER modellering is used in database design projects, data warehousing, and systems analysis to clarify data structures and support agreement on scope.

Limitations and note: While powerful for conceptual design, ER models may not capture all constraints or dynamic behavior; they should be complemented with detailed specifications and domain constraints.