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Entity-Relationship Diagrams

 

The informal definition of Entity-Relationship (ER) diagrams is directly taken from [19] in order to demonstrate the expressiveness of our specification language and the reasoning capabilities of our DL system. Figure 2 shows an example ER diagram specifying a relationship between a pilot and an aircraft. We assume a few primitive concepts (denoted in slanted font) and spatial relations (touching, containing, linked_with, text_value) representing geometrical objects (rectangle, circle, diamond, line, text) and their relationships.

  
Figure 2: A simple entity-relationship diagram

Connectors

A relationship-entity connection is a line that touches exactly one text label (expressing cardinality) and two other regions (rectangle or diamond). A cardinality is a text string with values chosen from the set .

An attribute-entity connection is a line that touches only two regions (circle or rectangle) and no text string.

Entities

An entity is a rectangle that contains its name. It touches at least one relationship-entity and optionally some attribute-entity connectors. It is linked with at least one diamond.

Relationships

A relationship is a diamond that contains its name. It touches one relationship-entity and optionally some attribute-entity connectors. It is linked with two entities.

Attributes

An attribute is a circle that contains its name. It touches one attribute-entity connector and is linked with an entity.



Volker Haarslev
Tue Jul 2 13:23:22 MET DST 1996