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An entity-relationship model describes identifiable entities, their database attributes and the relationships between them before committing to a physical database layout. Entity types classify the objects being recorded; relationship types describe associations, possibly with their own database attributes. Peter Pin-Shan Chen developed the model as a way to retain the meaning of data while relating several existing database models.^[ref]

For example, a laboratory records scientific instrument entities and measurement entities, with a relationship saying which scientific instrument produced each measurement. An scientific instrument's serial number is an identifying database attribute. A calibration date can describe an scientific instrument, while a usage duration can describe a particular relationship occurrence. Distinguishing these placements prevents unrelated facts from being conflated.

Cardinality and participation constraints add rules: every measurement might require exactly one scientific instrument, while an scientific instrument can produce zero or many measurements. An entity-relationship diagram records these choices. Translating the design to a relational schema introduces database relations, identifiers and appropriate foreign keys; the diagram itself does not require a particular index or file structure.

Peter Pin-Shan Chen proposed the entity-relationship model in his 1976 paper. It represents the application world through entities, relationships and attributes, together with a diagrammatic method for database design. The model aims to preserve semantic distinctions before translating them into a particular database representation.^[ref]

For example, a relationship between an employee and a project can carry an hours-worked attribute. That quantity belongs to the participation relationship, not to the employee or project alone. Making its placement explicit helps avoid duplicating or misinterpreting facts when designing a relational schema.

By Peter Pin-Shan Chen.

Primary source: weichselbraun.net/dbs/pdf/literature/chen.pdf

Sections 1–2.3, entities, relationships, attributes and identification.