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Data Modeling Made Simple with ER/Studio Data Architect by Steve Hoberman

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In a relationship between two entities, cardinality captures how many instances from one entity participate in the relationship with instances of the other entity. It is represented by the symbols that appear on both ends of a relationship line. It is through cardinality that the data rules are specified and enforced. Without cardinality, the most we can say about a relationship is that two entities are connected in some way through a rule. For example, Employee and Department have some kind of relationship, but we dont know more than this.

For cardinality, the choices are simple: zero, one, or many. Many (some people read it as more), means any number greater than zero. Each side of a relationship can have any combination of zero, ...

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