EXPRESS-G provides for two levels of modeling — a schema-level model and an entity-level model. Further, either of these may be complete or partial. This Chapter describes these various kinds of model. A schema-level model is one which displays the schemas, and the relationships between these schemas, that comprise an information model. The contents of the schemas (i.e., the entities, types, etc.) are not displayed. A schema level model, then, consists of: • The schemas that another schema Uses. • The schemas that another schema References. • The names of the things used and referenced. An example schema level model is shown in Figure 19.1. The Use relation is shown by a normal width relation line from the using schema to the used schema, with an open circle denoting the used schema end of the relationship line. The Reference relation is shown by a dashed relation line from the referencing schema to the referenced schema, with an open circle denoting the referenced schema end of the relationship line. The definitions used or referenced are shown as a list of names adjacent to the relevant relationship line, and connected to the relationship line by an arrowheaded line. If a definition is aliased, then this is indicated by following the original name of the definition by a ‘greater than’ (>) sign and the alias name. The model in Figure 19.1 consists of three schemas called fem, geom and mat. The fem schema Uses the property entity from the mat schema. It also References the point entity from the geom schema, and gives it the node alias. Schema-level models may be too large to fit on a single physical page. If this is the case then the page referencing symbols can be used to split the schema-schema relationship lines. The names of imported definitions should be on the same page as the importing schema symbol. An entity-level model is an EXPRESS-G model that represents the definitions and relationships that comprise a single schema. Thus, the components of such a model consist of type, entity and relationship symbols, together with role and cardinality information as appropriate.
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