EXPRESS-G has three basic kinds of symbol; defintion, relation, and composition. Definition and relation symbols are used to define the contents and structure of an information model. Composition symbols enable the diagrams to be spread across many physical pages. A definition symbol is a rectangle enclosing the name of the thing being defined. The type of the definition is denoted by the style of the box. Symbols are provided for EXPRESS simple types, defined types, entity types and schemas. The EXPRESS language offers a number of predefined simple types, namely Binary, Boolean, Integer, Logical, Number, Real and String. These are the terminal types of the language. The symbol for them is a solid rectangle with a double vertical line at its right end. The name of the type is enclosed within the box, as shown in Figure 18.1. The EXPRESS Generic pseudotype is not represented in EXPRESS-G as it is only used as a formal parameter to a function or procedure, and EXPRESS-G does not have these. The symbols for the select, enumeration and defined data type are dashed boxes as shown in Figure 18.2. • The symbol for a defined data type is a dashed box enclosing the name of the type. • The symbol for a select type is a dashed box with a double vertical line at the left end, enclosing the name of the select. • The symbol for an enumeration type is a dashed box with a double vertical line at the right end, enclosing the name of the enumeration. Although an enumeration is not a terminal of the EXPRESS language (because its definition includes the enumerated things), it is a terminal of the EXPRESS-G language. Figure 18.3 shows the symbol for an entity, which is a solid rectangle enclosing the name of the entity. The symbol for a schema is shown in Figure 18.3. It is a solid rectangle divided in half by a horizontal line. The name of the schema is written in the upper half of the rectangle. The lower half of the symbol is empty. EXPRESS-G does not support any notation for either function or procedure definitions.
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