Introduction
This document is an entity-relationship diagram, or “ERD,” for a system to manage electronic resources. An ERD is a model that identifies the concepts or entities that exist in a system and the relationships between those entities. An ERD is often used as a way to visualize a relational database: each entity represents a database table, and the relationship lines represent the keys in one table that point to specific records in related tables. ERDs may also be more abstract, not necessarily capturing every table needed within a database, but serving to diagram the major concepts and relationships. This ERD is of the latter type, intended to present an abstract, theoretical view of the major entities and relationships needed for management of electronic resources. It may assist the database design process for an e-resource management system, but does not identify every table that would be necessary for an electronic resource management database.
This ERD should be examined in close consultation with other components of the Report of the DLF Electronic Resource Management Initiative, especially Appendix E (Data Element Dictionary) and Appendix F (Data Structure). The ERD presents a visual representation of e-resource management concepts and the relationships between them. The Data Element Dictionary identifies and defines the individual data elements that an e-resource management system must contain and manage, but leaves the relationship between the elements to be inferred by the reader. The Data Structure associates each data element with the entities and relationships defined in the ERD. Together, these three documents form a complete conceptual data model for e-resource management.
Understanding the Model
There are several different modeling systems for entity relationship diagramming. This ERD is presented in the “Information Engineering” style. Those unfamiliar with entity relationship diagramming or unfamiliar with this style of notation may wish to consult the following section to clarify the diagramming symbology.
Entities
Entities are concepts within the data model. Each entity is represented by a box within the ERD. Entities are abstract concepts, each representing one or more instances of the concept in question. An entity might be considered a container that holds all of the instances of a particular thing in a system. Entities are equivalent to database tables in a relational database, with each row of the table representing an instance of that entity.
Remember that each entity represents a container for instances of the thing in question. The diagram below has an entity for “student” and another for “school.” This indicates that the system