The association between a forest and a tree with the cardinality is shown in Figure 2-2. In Figure 2-2 a line has been added to the many symbol and a circle has been added to the single symbol. One tree may be within a forest. One forest must contain many trees. The cardinality and association symbols used in the Standard are shown in Figure 2-4.
Attributes are the properties and characteristics of entities.
These are listed in the box with the entity name. This is an attribute that serves as an identifier for each unique record or entry in the entity and is called a primary key . An example of a completed model with the attributes is shown in Figure 2-5. Another kind of attribute is a foreign key . A foreign key is a primary key from another entity that points to a record in the related entity.