An_introduction_to_entity-relationship_diagram_notation

An_introduction_to_e - Entity-relationship diagrams Introduction Entity-relationship(or ER diagram notation describes the structure of data

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Entity-relationship diagrams Introduction Entity-relationship (or ER) diagram notation describes the structure of data important for a business. An ER diagram answers, in a concise and graphical way, the following questions: What kinds of data are relevant for the business? What are the business rules constraining such data? Consider, for example, the following ER diagram describing data relevant to a plumbing service provider. Even without learning the exact meaning of the notation, one can see that for the business in question employees, customers and jobs are of relevance, and that employees are assigned to jobs, while jobs are associated with customers. Notation Entity types In an ER diagram, a labelled rectangle represents a kind of an "object" or "thing" that is important for a business. For example, indicates that customers are important (for example, the business may wish to record information about customers in a database). We call such a "class" or "kind" of objects an "entity type". Note that the entity type "Customer" does not represent information about a particular customer, but just states that customers are relevant in general. Information about a particular customer is known as an "entity instance". Entity instances (information about specific customers) are not shown in an ER diagram. The specific customers (instances of "Customer") may come and go, but the fact that customers in general are relevant will remain unchanged. Customer Employee Customer Job does for
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Relationship types In an ER diagram, a line connecting two entity types (shown as rectangles) indicates that it is important to record connections between instances of the entity types. Let us consider the following diagram.
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This note was uploaded on 06/01/2010 for the course PN 178.328 taught by Professor Sue during the Spring '10 term at Massey Palmerston North.

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An_introduction_to_e - Entity-relationship diagrams Introduction Entity-relationship(or ER diagram notation describes the structure of data

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