POSC Specifications Version 2.3 |
How to Read ER Diagrams |
Entities appear as round-cornered boxes on the diagrams. The singular name of an entity appears in each box. On diagrams, entity names appear in upper cases with spaces instead of underscores between the words.
Moving the mouse pointer over the entity box will cause its definition and a list of related ER diagram names displayed in two separate frames.
Selecting (by clicking on the box) an entity on a diagram will bring up the entity dictionary.
Relationships are indicated by lines between entities. Associated with each line are verb phrases which define the relationship between the two entities. Because relationships are two-way, a phrase is defined for both ends of the relationship. Each verb phrase in an ERD is iconified as a small red and sensitive box:
The optionality of the relationship is indicated at each end of the line. A small circle indicates the relationship is optional for the connecting entity, meaning that an instance of the originating entity does not have to satisfy that relationship. The absence of the small circle indicates a mandatory relationship, which must be satisfied for instances of the entity.
Cardinality is indicated by the presence or absence of a "crow's foot" at the end of a relationship. If the terminating end of a relationship has a "crow's foot," an instance of the originating entity can be related to one or more instances of the terminating entity. If the terminating end is a single line, an instance of the originating entity can be related to only one instance of the terminating entity.
A relationship is a type of attribute pointing to an entity instead of a data type. An "inverse" relationship is defined that describes the relationship in the opposite direction.
A verb phrase of "void" indicates that the relationship, through drawn on the diagram, does not exist in the dictionary.
Class relationships are shown by the subtype entities being drawn inside the supertype entity, as illustrated by the diagram above.
Rules are no longer shown on the diagrams.