How to Read ER Diagrams
Information on these entity relationship (ER) diagrams describes entities (singular name) and relationships using the UML syntax as produced by Rational Rose 98. Express entities are represented as UML classes and relationships are represented as UML binary associations. The entity dictionary will contain additional information such as inheritance and rules. The illustration below shows what an ER diagram looks like. It also illustrates the hot link capabilities of the diagrams as described below.
Entities appear on the diagrams as a solid-outline rectangle with 2 horizontal lines inside the rectangle. The singular name of an entity appears in each box above the top horizontal line.
Pausing the mouse pointer over the entity box will cause its definition and a list of related ER diagram names displayed in two separate frames. Try this on the above diagram.
Selecting (by clicking on the box) an entity on a diagram will bring up the entity dictionary.
Relationships are indicated by a line connecting two entity symbols. Associated with each end of the line are an attribute name and a cardinality notation. These are located near the "to" entity. In the above example, entity document_specification contains attribute document which points to "0..*" instances of entity document.
The cardinality is indicated by a notation of the form "lower-bound..upper-bound" where lower bound and upper bound are integer values. In addition, the asterisk character (*) may be used for the upper bound denoting an unlimited upper bound (this corresponds to a question mark in Express). Examples include, "0..1", "0..*", "1..1" and "1..*".
The optionality of the relationship is indicated by the cardinality. A lower bound of zero indicates an optional attribute, meaning that an instance of the originating entity does not have to satisfy that relationship. A lower bound of one or more indicates a mandatory attribute, which must be satisfied for instances of the entity.
Supertype/Subtype relationships are shown by the UML generalization notation. This is a solid-line from the subtype to the supertype with an open triangle/arrow-head at the supertype end. There are two different forms that this can have with multiple subtypes shown on a diagram. A shared target style will have a shared segment (including triangle) to the supertype. This style is shown on the above diagram by the subtypes of document_specification. A separate target style is shown on the above diagram by the subtypes of document. Some diagrams will combine these styles.
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