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Unformatted text preview: Toturial for ER Diagram Entities Entities are the principal data object about which information is to be collected. Entities are usually recognizable concepts, either concrete or abstract, such as person, places, things, or events which have relevance to the database. Some specific examples of entities are EMPLOYEES, PROJECTS, INVOICES. An entity is analogous to a table in the relational model. Entities are classified as independent or dependent (in some methodologies, the terms used are strong and weak, respectively). An independent entity is one that does not rely on another for identification. A dependent entity is one that relies on another for identification. An entity occurrence (also called an instance) is an individual occurrence of an entity. An occurrence is analogous to a row in the relational table. Relationships A Relationship represents an association between two or more entities. An example of a relationship would be: employees are assigned to projects projects have subtasks departments manage one or more projects Relationships are classified in terms of degree, connectivity, cardinality, and existence. These concepts will be discussed below. a relationship set or relationship type is a set of associations between the members of two or more entity sets degree of relationship – • number of entity types participating • binary relationships 2 participating entity types Employee works for Department • ternary relationships 3 participating entity types...
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- Fall '09