Dlf Electronic Resource Management Initiative

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Report of the DLF Electronic Resource Management Initiative Appendix D: Entity Relationship Diagram for Electronic Resource Management 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…show more content…
“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 ( ( ) indicates that the relationship is optional—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 mandatory—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” ([pic] ) indicates that many such relationships between instances of the related entities might exist. The following diagram indicates all of the possible combinations: [pic] 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: [pic] It is important to note that relationship optionality and cardinality

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