An EXPRESS specification can be described using graphical notations known as EXPRESS-G. The specification for EXPRESS-G is set out in Annex D of Part 11 of STEP. EXPRESS-G may be used as a data specification language in its own right. In other words, there is no requirement to have associated EXPRESS specification.
There are three main classes of EXPRESS-G notations as summarized in Table 1.
Table 1: A summary of the three main classes of EXPRESS-G notations.
Definitions | Notations used to represent EXPRESS simple, named and constructed data types, and schema declarations. |
Relationship | Notations denoting logical coupling between definitions. |
Composition | Notations to enable a diagram to be displayed on more than one page. |
EXPRESS-G supports simple data types, named data types, relationships and carnality. EXPRESS-G also supports the notation for one or more schemas. It, however, does not provide any support for the constraint mechanism provided by the EXPRESS language.
An EXPRESS-G specification is presented as a collection of notations known as a diagram. An EXPRESS-G diagram may be used to display data abstraction at: entity level or schema level. An EXPRESS-G diagram may also span more than one page.
For illustrative purposes, Figure 1 and Figure 2 show an EXPRESS-G diagram for a single EXPRESS schema. The schema is an example of an abstraction of a person. The graphical diagram is divided into two pages to illustrate the use of multiple pages. The example abstraction assumes that a person has certain defining characteristics, including a first name, a last name, an optional nickname, date of birth, and a description of their hair. A person is either male or a female. A male may have a female wife; in which case the female has a male husband. A person may have children who also are persons.
Figure 1: Page 1 of 2 of a complete entity level diagram of an abstraction of a person.
Figure 2: Page 2 of 2 of a complete entity level diagram of an abstraction of a person