Chapter 2 Data Models
Chapter 2Data Models
Answers to Review Questions
1. Discuss the importance of data modelling.
A data model is a relatively simple representation, usually graphical, of a more complex real world object event. The data model’s main function is to help us understand the complexities of the real-world environment. The database designer uses data models to facilitate the interaction amongdesigners, application programmers, and end users. In short, a good data model is a communicationsdevice that helps eliminate (or at least substantially reduce) discrepancies between the databasedesign’s components and the real world data environment. The development of data models, bolstered by powerful database design tools, has made it possible to substantially diminish thedatabase design error potential. (Review Section 2.1 in detail.) 2. What is a business rule, and what is its purpose in data modelling?
A business rule is a brief, precise, and unambiguous description of a policy, procedure, or principle within a specific organization’s environment. In a sense, business rules are misnamed: they apply to any organization -- a business, a government unit, a religious group, or a research laboratory; large or small -- that stores and uses data to generate information. Business rules are derived from a description of operations. As its name implies, a description of operations is a detailed narrative that describes the operational environment of an organization. Such a description requires great precision and detail. If the description of operations is incorrect or incomplete, the business rules derived from it will not reflect the real world data environment accurately, thus leading to poorly defined data models, which lead to poor database designs. In turn, poor database designs lead to poor applications, thus setting the stage for poor decision making – which may ultimately lead to the demise of the organization. Note especially that business rules help to create and enforce actions within that organization’s environment. Business rules must be rendered in writing and updated to reflect any change in the organization’s operational environment. Properly written business rules are used to define entities, attributes, relationships, and constraints. Because these components form the basis for a database design, the careful derivation and definition of business rules is crucial to good database design.
3. How would you translate business rules into data model components?
As a general rule, a noun in a business rule will translate into an entity in the model, and a verb (active or passive) associating nouns will translate into a relationship among the entities. For example, the business rule “a customer may generate many invoices” contains two nouns (customer and invoice) and a verb (“generate”) that associates them.

4.
What does each of the following acronyms represent, and how is each one related to the birthof the network data model?a.CODASYL
Conference on Data Systems Languages. This group created a COBOLstandard; furthermore, CODASYL created the network model specifications.
b.SPARC
Standards Planning and Requirements Committee. This committeeaugmented the database standards in 1975.
c.ANSI
American National Standards Institute. Adopted the CODASYL database specifications as a standard database mod ell.
d.DBTG
Database Task Group. This group defined an environment to facilitate database creation and data manipulation.
5. What three languages were adopted by the DBTG to standardize the basic network datamodel, and why was such standardization important to users and designers? The three languages were:1.
The
DDL (schema)
Constitutes the Data Definition Language for the database schema. The DDL's use enabled the database administrator to define the database schema, i.e., its over-all blueprint.
2.
The
DDL (subschema) allows the definition of the specific database components that will be used by each application.
3.
The
DML
is the Data Manipulation Language that allows us to manipulate the database contents.Standardization is important to users and designers because it allows them to shift from onecommercial application to another with little trouble when they operate at the logical level

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