Different versions of IE exist, with no single standard. In one form or other, IE has long been supported by many data modeling tools, and remains one of the most popular notations used in industry for database design. The metamodel presented here provides one of many possible ways to model the IE notation. Feedback and alternative suggestions are welcome. Entity types and attributes The IE approach depicts entity types as named rectangles.
Some versions support basic constraints on attributes.
For example, an attribute that is part of its entity type's primary identifier might be underlined, and whether an attribute is mandatory might be recorded off the diagram. Although no standard notation exists for these constraints, I'll include them in the metamodel. Figure 1 Entity type and attributes Assuming that it is recorded whether an attribute is mandatory, and that primary identifier attributes must be mandatory, one way of metamodeling this IE fragment in ORM is shown in Figure 2. The subset constraint may also be displayed simply as a dotted arrow. Relationships are typically restricted to binary associations only, which are shown as named lines connecting the entity types.