11 - er-diagrams

11 - er-diagrams - The Entity Relationship Model Entity...

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Page 1 The Entity Relationship Model Entity Relationship Diagrams The ER (entity-relationship) data model views the real world as collections of basic objects (entity sets) and relationships among these objects and is intended primarily for the DB design process. Entity sets are sets of objects that share common characteristics or attributes. The members of entity sets are called entities. We can express the overall logical structure of a database graphically with an ER diagram. Its components are: rectangles representing entity sets. ellipses representing attributes. diamonds representing relationship sets. lines linking attributes to entity sets and entity sets to relationship sets. Lines may be directed (have an arrow on the end) to signify cardinalities for relationship sets. Cardinalities express the number of entities to which an entity can be associated via a relationship. The arrow head points to the entity that ``one'' refers to. The above is a ER diagram of a simple customer account database. It consists of two
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11 - er-diagrams - The Entity Relationship Model Entity...

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