Entity-Relationship Diagram
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| Definitions |
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Entity
An entity is a person, place, thing, concept, or event about which it isnecessary to keep data. It is represented by a rectangle; the entity name iswritten in capital letters. |
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Relationship
A relationship is a named business association between two entities. Therelationship is represented as a line between entities; its name is written inlower-case letters. Names are written in both directions - aboveand below the relationship line. |
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Entity-Relationship Diagram (E/R or ERD)
An Entity-Relationship Diagram is a means of showing a "point-in-time"snapshot of how data is organized. An ERD is not a flow diagram. AnERD may show all or part of an enterprise's data. |
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Reading the diagram |
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Start at either entity and read the relationship clockwise like a sentence. For example, in this E/R Diagram, read "CUSTOMER places ORDER" and "ORDER is placed by CUSTOMER." |
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Cardinalities |
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The marks on either end of the relationship line tell the cardinalities of the entities: how many occurrences of the entities participate in the relationship. |
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For each entity in a relationship, these are the possible cardinalities: |
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There must be one and only one occurrence of the entity. |
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There can be zero or one occurrence of the entity. In other words, therelationship may not always be created between the entities. |
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There can be zero or more occurrences of the entity. Again, therelationship may not always exist. |
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There can be one or more occurrences of the entity. |
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This relationship would be interpreted as "A CUSTOMER placeszero or more ORDERS" and "An ORDER is placed by only oneCUSTOMER." |
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