Erd

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 being modeled may contain one or more students and one or more schools.

So far, no relationship between students and schools has been indicated.

Relationships

Relationships are represented by lines between entities. Relationship lines indicate that each instance of an entity may have a relationship with instances of the connected entity, and vice versa.

The diagram above now indicates that students may have some relationship with schools. More specifically, there may be a relationship between a particular student (an instance of the student entity) and a particular school (an instance of the school entity).

If necessary, a relationship line may be labeled to define the relationship. In this case, one can infer that a student may attend a school, or that a school may enroll students. But if necessary, this relationship could be labeled for clarification:

Read the first relationship definition, “attends,” when tracing the relationship left to right or top to bottom. Read the second definition, “enrolls,” when tracing the relationship right to left or bottom to top.

Optionality and Cardinality

Symbols at the ends of the relationship lines indicate the optionality and the cardinality of each relationship. “Optionality” expresses whether the relationship is optional or mandatory. “Cardinality” expresses the maximum number of relationships.

As a relationship line is followed from an entity to another, near the related entity two symbols will appear. The first of those is the optionality indicator. A circle ( Ð'™ ) indicates that the relationship is optionalвЂ"the minimum number of relationships between each instance of the first entity and instances of the related entity is zero. One can think of the circle as a zero, or a letter O for “optional.” A stroke ( | ) indicates that the relationship is mandatoryвЂ"the minimum number of relationships between each instance of the first entity and instances of the related entity is one.

The second symbol indicates cardinality. A stroke ( | ) indicates that the maximum number of relationships is one. A “crows-foot” ( ) indicates that many such relationships between instances of the related entities might exist.

The following diagram indicates all of the possible combinations:

In our model, we wish to indicate that each school may enroll many students, or may not enroll any students at all. We also wish to indicate that each student attends exactly one school. The following diagram indicates this optionality and cardinality:

It is important to note that relationship optionality and cardinality constraints apply specifically to the system being modeled, not to all possible systems. According to the example modeled above, a school might not enroll any studentsвЂ"that relationship is optional. A school without students is not much of a school, and indeed if the system being modeled were a school system enrollment database, the relationship would probably be mandatory. However, if the system being modeled is an extracurricular honors program, there may be schools that have no students currently participating