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

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A rule can be either a data rule or an action rule. Data rules are instructions on how data relate to one another. Data rules (also known as cardinality rules) define the quantity of each entity instance that can participate in a relationship. For example:

·         Each product can appear on one or many order lines.

·         Each order line must contain one and only one product.

·         Each student must have a unique student number.

Action rules, on the other hand, are instructions on what to do when attributes contain certain values:

·         Freshman students can register for at most 18 credits a semester.

·         A policy must have at least three claims against it to be considered high-risk.

·         Take 10%

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