IEvsORM[1] - Entity Relationship modeling from an ORM...

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Entity Relationship modeling from an ORM perspective: Part 3 Terry Halpin Microsoft Corporation Introduction This article is the third in a series of articles dealing with Entity Relationship (ER) modeling from the perspective of Object Role Modeling (ORM). Part 1 provided a brief overview of the ER approach, and then covered the basics of the Barker ER notation. Part 2 completed the examination of the Barker ER notation by discussing verbalization, exclusion constraints, frequency constraints, subtyping and non- transferable relationships. Both parts compared the Barker notations with the corresponding ORM notations This article discusses the Information Engineering notation for ER, relating it to relevant ORM constructs. The Information Engineering (IE) approach began with the work of Clive Finkelstein in Australia, and CACI in the UK, and was later adapted by James Martin. Different versions of IE exist, with no single standard. In one form or other, IE is supported by many data modeling tools, and is one of the most popular notations for database design. Entity types, attributes and associations In the IE approach, entity types are shown as named rectangles, as in Figure 1(a). A ttributes are often displayed in a compartment below the entity type name, as in Figure 1(b), but are sometimes displayed separately (e.g. bubble charts). Some versions support basic constraints on attributes (e.g. Ma/Op/Unique). Figure 1 Typical IE notation for (a) entity type and (b) entity type with attributes Relationships are typically restricted to binary associations only, which are shown as named lines connecting the entity types. As with the Barker notation, a half-line or line end corresponds to a role in ORM. Optionality and cardinality settings are indicated by annotating the line ends. To indicate that a role is optional , a circle “ ! ” is placed at the other end of the line, signifying a minimum participation frequency of 0. To indicate that a role is mandatory , a stroke “|” is placed at the other end of the line, signifying a minimum participation frequency of 1. After experimenting with some different notations for a cardinality of “ many ”, Finkelstein settled on the intuitive crow’s foot symbol suggested by Dr. Gordon Everest. In conjunction with a minimum frequency of 0 or 1, a stroke “|” is often used to indicate a maximum
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IEvsORM[1] - Entity Relationship modeling from an ORM...

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